In the 1970s APIs were called modular programming. It was the essence of Unix.
Problem is, the teenagers who implement a lot of the gizmos on the Web care more about making their sites look pretty than providing information to others.
Besides this, there is a widespread attitude that "no robots must be allowed in", ie under no circumstances should other sites be allowed to build on "my" information.
For example, when Jamie Jones and I were implementing istopbrexit.info, I thought "we've got the user's postcode and that of this meeting, let's tell them how to get there". A site called traveline.info seemed to be providing passable public transport information, but I couldn't see how to feed postcodes to its interface.
When I enquired, they told me I had to pay £500 for a license. This is a pretty stupid attitude, because if they provided a simple API, more people would find out about their site (and see their ads).
As for Google maps, it's only for Californians who never get out of their cars. It lacks any detail for pedestrians. streetmap.co.uk is far superior in Britain and increasingly openstreetmap.org across the globe.
(We took istopbrexit.info down when the disaster happened, but also because the major anti-brexit organisations were determined to compete with me instead of cooperating, and for personal reasons. Unfortunately, the domain name was taken over by a porn site.)