The devs read stuff and care about feedback/suggestions. However, there are limits to what they can and will do even when they are aware of a suggestion (and -- let's face it -- the volume of posts is such that they can't reasonably read everything).
- Some suggestions are made by people that simply don't understand the game or what it is modelling. For example, "I should never be considered a tyrant for revoking a title from one of my vassals, since all the land in my realm belongs to me!" shows a lack of understanding of how feudalism works, so that suggestion would probably not go anywhere.
- Some feedback is unconstructive and/or rude. For example, "This mechanic is bad, and the person that made it should be fired!" doesn't tell them how something can be improved, and even if improvements were suggested I'd imagine the rudeness wouldn't exactly make the devs eager to please that person.
- Some suggestions are simply bad. For example, there are people that don't know history half as well as they think they do that suggest ways they think would make the game more accurate but that would have the opposite effect, and there are people that simply want to make their preferred playstyle/region/religion/culture/etc. better and that don't care at all if their suggestion would make things a lot worse for others (e.g. someone might suggest things that'd make playing as an independent ruler a better experience but that very obviously would screw over vassals to a massive extent and not see a problem with the suggestion because they have no interest in playing as a vassal).
- Some feedback is contradictory. For example, one person might feel that expansion should be easier while another might feel it already is far too easy to expand, and you obviously can't please both of them at the same time (unless you lock blobby CBs/anti-blobbing mechanics behind a game rule, but game rules aren't always a good answer).
- Some suggestions are too far removed from the devs' vision of what the game should be. For example, the game is very clearly designed around the idea that you play as a landed character belonging to a dynasty and keep playing as that character (or their dynastic successor) regardless of whether you lose your primary title, so a suggestion to move to something similar to EU4 where you keep playing as the same country regardless of which dynasty is in charge would likely not go anywhere.
- Some suggestions are unfeasible. For example, Total War-style battles would quite possibly be nice (at least if you like that kind of thing; I'd imagine not everyone does), but the game really isn't designed with something like that in mind, so even if the devs potentially might agree that it's a cool idea it's probably never happening.
- Some suggestions can't be dealt with in the immediate future due to being too large to get done alongside other things that are planned and/or due not being reasonable priorities based on what else is planned. For example, if someone had come up with a suggestion related to India while Northern Lords was being developed that the devs liked, it's possible it'd still have to wait if it was too large since it really wouldn't have been related to the theme of the expansion.
- Related to the above, a lot of people have a poor sense of both priorities and how long something would take to do. Some things that are being suggested that would require a fairly small amount of work to get done simply aren't as urgent to deal with as other things -- big or small -- and thus will likely be delayed until they are deemed a priority (or until some dev that feels strongly about the matter can find the time to get something done), and it is not uncommon for people to claim something would be trivial to do (e.g. "It'd take a dev five minutes to do this!") when it really wouldn't be.
- The devs doubtlessly have their own ideas as to what is and isn't right for the game (based on both personal preferences, any internal design documents that exist, and data available to them), and they might simply feel that what is being suggested is going to make the game noticeably worse in some regard (e.g. unacceptably inaccurate in some regard, terribly balanced in some regard, much slower to run, or less enjoyable without sufficient justification). At times, the devs' ideas might turn out to be not so great (and at times the community might correctly identify that something isn't a good idea when it is first presented), but there will also be times when the devs have ideas that turn out very well (possibly even in cases where the initial feedback was negative).
A related thing to note is that dev responses easily can be misconstrued in various ways, e.g. a "This is a cool idea!" meant to be read as "I personally like it, and maybe we'll do something like this at some point in the future!" might be taken as "I'll get started on this right away, ensure it gets implemented exactly the way you envision it, and promise it will be in the next patch!" (which tends to make the people taking it that way upset if the devs fail to deliver on what they see as a promise), and a "Sorry, but no" might be taken as "The devs hate [insert culture/religion/country/group/etc. related to the suggestion]!" (I suspect everyone here has seen threads like that pop up every now and then...). This might occasionally make the devs reluctant to comment on suggestions if they don't feel strongly about them and aren't prepared to make a promise regarding something, particularly if the devs reading a suggestion have run into that issue recently or believe (correctly or not) the person making the suggestion (or someone that's vocally supporting or opposing the suggestion) is someone they recognize as a person that previously misconstrued a dev statement and got upset.
