Because as you skip pages with most pagination implimentations the DB has to scan more index keys. If they allowed paginating so far, without some kind of expensive preaggregation, you could easily create a DDOS attack.

?afterId=x will perform much better than page based pagination and doesn't have this problem as much, but it can still cause undesired io and cache activity in the DB when you start paginating a lot to unpopular content.