Because paradox titles are essentially sandboxes. You can do whatever you want.
Nope. Paradox games are essentially historical(-ish) simulators (whatever shallow they are, they still are the best on market). You can do whatever you want within rulesets.
In CK2 you can't swap to "normal" inheritance as a tribal. You can't hold churches as a catholic without penalties. You can't play Pope.
I created that juggernaut from a single county as hellenic, so yea I have every right there is to play as an invincible monster.
Of course, make a custom empire based on Thrace or Greece, and you're good!
However if you have an elective and the penalties for underage people is so severe (even your most loyal people wouldnt support you) so you have to bribe your commanders who cant even hold a sword, something is wrong I'm sorry.
No, nothing is wrong. At least I can't see anything wrong here. Yes, to put a child on throne when every commander have a reason to believe he can take a throne for himself, you need to invest into. Absolutely reasonable.
Bedause of the whole bloodline thing, choosing an heir became even more imperative woudlnt you agree? Also you are forced to matrinially marry to get them into the dynasty.
You see, that's a difference.
Yes, selecting a heir is imperative. So it's a thing you should invest into, work about, and it should be hard to always get a heir you want. You should use favors, please people you need, maximize your own voting power. You should work hard to select a heir you want to
because it's imperative, not the opposite way.
You can't really declare it's not possible to put a child into Roman throne. I modded Imperial Government to my custom empire and played it a lot. I can place there a heir I want, until my ruler has serious problems; then it's my prefered heir siblings. It take my deal of work, sure.
I only reformed byzantium for the achievement and the looks
Yes, and you're supposed to pay (play designed way) for this achievement and the looks.
I mean, let's differ different lines of this arguement.
1. "Hey, they forbid me to do anything I want". Yes, they do. That's ok.
2. "Hey, mechanics they created aren't working!" They are working and they do it's job.
3. "Hey, mechanics they created aren't fun!" They're fun for me. Eldership and Imperial Elective (and rework of elective governments as a whole) are the best things happened with CK2 (as a simulator game). If they're not fun for you, well, it's a pity, but it's a question of personal preferences, and that's why there are a lot of other countries with different rulesets out there.