since everyone else is talking about build orders
the problem with build orders is if your build order isn't as good as your opponents, which is different on every specific map (oh you missed 1 reclaim wreck here, you didn't take this path but a longer one, you should have sent transport here earlier, etc etc), you will not be starting on even ground and will be at a disadvantage from beginning and for the rest of the game in a constant uphill battle, and next game you will get a different map with different optimal build order that you will fail in a different way for same result
so you have to get past (master) build orders to even start playing the game itself, which are specific to each map, maybe it is more like "openings" in general rather than build orders that I mean, it is not just about what buildings you build but everything else you do in beginning, such as the path your engies take and what/whether they reclaim in between, which definitely isn't the same on every map
all the "pro" players that are saying otherwise are simply too good to notice what they even do in beginning of the game because it is second nature to them and just seems "normal", meanwhile to those that don't it is like their opponent starts better off than them and they have to play catch up instead of the game, all because they didn't memorize what to do in beginning of this specific map
in mapgen (didn't play yet so idk) it seems this is removed because no one knows where mexes or reclaim will be to have a predefined optimal opening for the specific map (because it never existed before) to gain immediate advantage over someone who doesn't, they both have to figure it out on the fly instead of one of them going "ok I should take this path with this engi, don't forget to send a raid to here, no one expects it, this path with this engi, reclaim these wrecks in between..." while other goes "what even is this map, I better expand here, oh wait there were wrecks in other direction, where did those labs come from?..."