a) all-in-one is often a compromise, e.g. people might like a a relatively plain Linux or BSD to do routing - best on "boring" hardware, but you don't get the best WiFi performance on such hardware. Whereas APs/integrated routers don't necessarily run such OSes best, have driver issues for newest standards etc.

b) incoming internet connection is often in a corner somewhere. Best position for an AP is elsewhere. If that's a problem depends on how you get internet and what the place looks like.

Routers are security sensitive and are long lived. Much like the unix philisophy, do one thing and do it well. A GigE router has been plenty for many years and will likely be fine for most people for many more. Ideally routers are kept as simple as possible and as secure as possible.

WIFI APs however are changing quickly, beam forming, MIMO, 802.11ac, 802.11n, wifi6, 802.11g, etc. Not to mention ideally APs are placed for optimal propagation and might even having more than one per home.

So ideally you don't have to throw away your router to get the current gen WIFI or to add a second AP. Similarly with separate AP and router you don't have to throw away your AP because you need more/faster ports, better buffer bloat mitigation, or improved QoS on your router. When troubleshooting it's really nice to be able to tell if it's an AP problem or a router problem.