There is not much rationalization worth doing on this regard for anyone that seriously played the game, diplomacy is just wonky right now.
It's very hard to get relevant alliances. Even if you look at the AI, you'll see it will generally mostly have irrelevant and nonsensical alliances. In my current game, I got extremely lucky since the US (which got is *** handed to it 4 times in a row now) has locked its only alliance slot in an alliance with haiti... which does them no good and abandon them each and every time anyway. To get relevant alliances, Genial or Protective + bankrolling is mandatory. It's fun giving 100k/week to a protective GB to get an alliance. And you got to time it extremely well because if you build a decent military, the AI will eventually get cautious of you and you won't be able to get any military pact with them, even at 100 reputation and a long history of fighting side by side.
Alliances are simply worth less than the boost to Infamy loss of unspent diplo mana, since your allies will most likely abandon you anyway. Last example, Joseon asked my Mexico for an alliance. I figured, why not since I can't get any relevant country to ally with me. I declared on an independent Hudson Bay company, which had a grand total of 3 outdated battalions against my 150+ state of the art force, and Joseon abandoned me right away...
Defense pact are the real tool, they are slightly easier to get (still need bankrolling) and AI seems less likely to bail out on you. But it has a limited use.
Problems are imho that AI has no memory and value getting obligations way too much for the use it has of it. You can be the most faithful ally ever, help your ally countless times and it will probably have no problem joining side against you for an obligation from luxembourg. Obligations are very valuable and it can be a game changer to be able to call someone on your side during a war or quickly boost relation to 80, that's one of the reason why bankrolling is by far the most powerful non-aggressive diplomatic interaction, but AI hardly know how to use it and it seems it makes no difference between getting an obligation from lanfang or from GB.
I think the intent is to avoid game lasting unbreakable alliances which lead to stalemates like in Stellaris, because alliances in this era (at least concerning the GPs) were more pragmatic and subject to change and gameplay wise right now, a game lasting alliance between GB and France could just kill the game for everyone else. That's why why have randomized evolving attitude. But right now alliances are useless, while you can see undying nonsensical game lasting defense pacts. And since you can only have one alliance for a good chunk of the game, the AI will often lock itself in poor and unreliable alliances.
Edit: After giving some though to it, one thing severely lacking is a boost to acceptance for common rivalry/enemies. While you have common enemies it's a very "real politik" thing to be allied, but as soon as you or the other part aren't rivaling anymore, the alliance would stop with the loss of acceptance points.
tl;dr; I think the fact that you can't get game lasting bros is intentional, but the AI diplo behavior is also all over the place.