Don't try to excuse things. Paradox games have never been exclusively oriented around multiplayer, and winning vs. AI is never an expected outcome in most good 4Xes unless you play on pussy difficulty levels or cheat.
The fact that player diplomacy can occur outside of the structure of a game's diplomatic system doesn't excuse having a shitty, bare-bones diplomatic system. Aurora has a more in-depth diplomacy system, and that's literally just a set of different factors which result in your diplomacy score with a race going up (and eventually allowing trading, alliance, &c.) or down (resulting in war). Same shit as Stellaris, but there's no expectation of anything more fleshed out and not really any need for it. By contrast, Stellaris is also just a set of different factors which result in your diplomacy score with a race going up (eventually allowing... disadvantageous alliances and federations with the shitty little powers) or down (resulting in them thinking that you're mean).
Stellaris diplomacy is bad enough that a system where the expected outcome of all diplomatic encounters is either a slow, inevitable spiral into a war of annihilation or a de facto agreement of non-interference is more interesting and more functional. That's what I'm saying. All of the little nuances that made diplomacy, DoWs, and peace treaties palatable in other Paradox games are missing. No espionage, nothing beyond alliances/feds and the shittiest implementation of research agreements I've ever seen (since the AI basically never grants any of the other simple agreements that were included), incredibly limited options for DoWs and peace deals, and a lack of the standard historical context to help you fill in the gaps for all this shit.
Poland negotiating a royal marriage with Portugal and allying with the Ottomans for a war against the Papal State may not be interesting mechanically in EUIV or whatever, but the informed context adds some quality to it. Nobody ever gave two shits about the Republic of Gor'Blax entering a research agreement with the Mary Sue Confederation in which one or both might potentially get a bonus to their research... on a single tech... if they happen to roll a tech card which has already been researched by the other... within the brief time span of the agreement in which they will likely only research five or six new technologies. >.>