Well you know what: I am generous. I will tell you why your assumption that "Larian just does it because they think poeple are stupid" is in fact probably wrong.
You are right that a physical playtable and a PC game are 2 different things. You are also right that a PC has no serious problem with managing more players, NPCs and monsters, where a human irl would probably struggle. So, yes. There is no real problem handling encounters for bigger parties. Why not make them then?
You still have to have some kind of maximum party size. Okay but why the miserly 4? "Its so unfair!! I am smart enough to control so many more!". While there is combat to consider you also have to consider out of combat situations. And every PC more is a significant boost in ability-versatility, class features and everything else that PC brings to the table. While you could make combat encounters dynamic enough to account for any reasonable party size, you are really boosting everything else outside combat. And those things are not really that easy to balance. This is a big advantage for an irl table and mayor disatvantage for PC-Games. A DM can account for the very specific party he knows he is playing with. Can make additions and hurdles to a campaign to account for more player power in general. 5e classes and design are also not comparable to older editions which were used in the other games. With 4 players you can cover pretty much all things you would need adventuring, if you want. With more players you get abundance fast.
While a lot of those things can be juggled around for more than 4 players, I still think the reasoning for not doing the mechanical changes in this department is not
the assumption that your audience is made by smooth-brained people who can't keep up with the complexity of having more
as Tuco put it. The reason here is much more probably: its easier to design encounters and a world for a fixed party size and 5e's default is 4. (Some of the weird homebrew from Larian notwithstanding, its more plausible to have the default as a fallback, instead of chaning *all* the things).
The other points are: campaign building and story telling. Like it or not: but Larian seems to want to give every companion a unique story and interactions. In short: make them really great characters and bring a story for all of them to life. If you wanted to have a max party size where you still have relevant story and things like members leaving because of the descisions then you potentially have to make just true neutral characters which are kinda bland, or you have to do more origins to account for potential loss, while still maintaining the 6 PC party size. Seems like reasonable reasoning to me. Look at the older games, which had more companions: none of them were half as developed as they are today (which is fine, but its still a huge difference).
Long story short: Tuco's assesment, that Larian just thinks its players too stupid is itself not the brightest argument, as other reasonings are way more likely.
You *could* do it all, but why? So you have just a little more of everything and in some parts an easy mode? Don't worry Tuco, I know some people are not good at working with limitations. Its hard. And that is okay, the modding community will have your back and help ya out
