Which is a natural function of an increase in the number of pages you like and the number of friends you have, an in crease in the number of pieces of content they produce and your non-increasing attention span. You will see less of content from any one source as a function of time. The source can work around this by producing more interesting content that the algorithm decides you're more likely to want to see or by paying to have their content shown to you.
If you got them all you would be aware of what you had liked but was sending you too much uninteresting stuff and you would unlike them or you could organise them to either 'get everything' or 'highlights only'.
FB's non-optional control and filtering of the feed is to benefit them and give them a monetization option, it isn't a service to you.
