I would say that it's roleplaying/game mechanics and QoL/UI decisions shouldn't mix. The issue is that a 4-person party is a developer QoL/balancing decision, yet characters in-game reference it, making it a part of the BG3 world. Essentially, they're breaking the 4th wall but in a very weird way. Most characters that break the 4th wall know they're doing it (e.g., Deadpool). BG3 companions, however, clearly aren't aware of the real world and play it straight, which causes dissonance.
Why is my party "full up" at 4? The real answer is "because the devs wanted to craft an experience for a party of 4." The Lae'zel answer is...[unknown]?????