Without the "fragmentation" the problem of third parties censoring applications would be even worse and it still wouldn't be any easier to deploy anything. If you want a proprietary centralized service loaded with malware, there are already plenty of those to choose from on other systems, complete with their own gatekeepers.

Neither MacOS nor Windows prevent you from installing any application you desire[0], direct from the vendor. That has only recently begun to change thanks to app stores, which bear a remarkable similarity to package repos.

[0]Ok, there are some hoops regarding unsigned drivers.