It's how hyped it was and expectations set by Skyrim. Starfield was seen as the next step on from Skyrim in terms of game scale, and Bethesda hyped it up as their biggest and best game ever. It's neither of those things.
Also frankly in terms of RPGs, it feels dated. Witcher 3 set a new bar for what an RPG should be, but Starfield doesn't seem to have learnt those lessons. Baldurs Gate 3 has also set a high bar for RPGs this year, and Cyberpunk 2077 (for all its own flaws) also set a high bar for RPGs.
Starfield is an ok game but when it's hyped as it going to be the greatest game ever from Bethesda and going to be biggest game of the year, I'm not surprised it's being shat on when it turns out it's not.
But hopefully Starfield will be an important bump on the road for Bethesda. Bigger is not necessarily better and hopefully that lesson will carry in to Elder Scrolls VI.