Privacy is one part. The cost of joining & maintaining a server on the network is the other. Many servers have shut their doors due to expensive hosting. If you are lenient on how many messages need to actually be stored on the server for archives, self-hosting is now much more accessible which leads to a healthier, more decentralized network since more nodes can afford to join.

You can still creating your own archives in your clients or on your specific server via s2s communications, but Matrix has this as a network requirement for eventual consistency.