Mongo licensing prevents DO doing this themselves, although they could partner with them.

Currently using Mongo Atlas free tier for a side project - it's been pretty wonderful so far

You can get something with a MongoDB API if you use Microsoft's CosmosDB, although it is very expensive and not actually Mongo underneath. But it has some pretty impressive technical specs too, especially geographically distributed multi-master writes and impressive response times. I don't work for MS and haven't used the Mongo API, just aware of it.