It's because the TOS of Google Maps only allow you to consume their imagery and maps using their API. See Section 3.2.4 e) from https://cloud.google.com/maps-platform/terms/#3.-license-
(e) No Use With Non-Google Maps. Customer will not use the Google Maps
Core Services in a Customer Application that contains a non-Google
map. For example, Customer will not (i) display Places listings on a
non-Google map, or (ii) display Street View imagery and non-Google
maps in the same Customer Application.
If you don't, you are already doing something risky or maybe illegal (if in a pro context at least)
You also need to be aware that Google Maps is also not the cheapest solution as in recent years, the cost to run a web map application using Google Maps skyrockets (moreover now, even for free tiers, you need a credit card). The other sources cost really less and for most cases are enough and in some cases they also are better for example for bike maps when using OpenStreetMap.