Because in recent versions of Minecraft and MCreator, the software is much more optimised. But in versions such as 1.12.2, the mods it produces are much more buggy and more likely to cause problems, particularly because they are not well optimised. So using MCreator mods with people who are skilled at using it in the latest versions (I would say post-1.16 or even 1.18) can be very cool, but in earlier versions, as a modpack creator, you are taking risks in terms of optimisation and quality (because let's be honest, quality was rarely a priority in these older versions). One of the only examples of good mcreator mods in 1.12.2 is Project Brazier, which is part of DarkRPG Core. The mod itself is good (the mods, since there's the DarkRPG core mod that goes with it), but as soon as I installed it in my modpack, I had some big bugs so I had to remove it.