Because then you get the best of both worlds, powerful routing hardware that can easily route and firewall at multi gig speeds, extreme flexibility in software packages to run on your open router platform, and a prosumer AP with best in class wifi performance, antenna configuration, mimo, solid chipset and driver, etc.
Doing everything on a prosumer router running mips or arm with limited package selection at best and a locked down router is at worst is subpar just as trying to get good modulation rates with a client oriented wifi card running in AP mode with subpar antenna configuration.
If you want the best wifi and the best routing/firewall/IDs with the widest package selection (and ability to just run any x86 application) then a separate router box running an x86 based os like opnsense,pfsense,whatever paired with a high end AP (business AP is a good choice) will always be the way to go unless you value compact, low power, or simplicity over achieving the best performance.