Long time Mac user here. We are used to Mac versions of software following PC releases. Macs are just more complicated to program for. I've been playing BG3 EA on Steam for a month (?) now and love it. I really don't mind that it's only act one. I get to try out multiple builds and parties and see how the game changes. There is a HUGE benefit to going last in that Larian will have more time to improve this great game even further with storyline, gameplay and other improvements on the PC and console releases. By the time the Mac version comes out, the game will be at its best.
(Im hoping to get some sleep somewhere around October....)