Instant messaging is more complicated than you'd think. Sending a message from one device to another seems simple, but it's hard; especially on mobile. NAT, Firewalls, smartphone battery optimisation are all technical challenges, but then you also got user expectations. Users expect to see the same messages in the same order on every device. They want to be able to send photos, files, locations, voice messages and much more. On top of all that, full end to end encryption is a must have nowadays. Especially if you let average people run their own servers.
Matrix seems overly complex, but once you attempt to reinvent it, you end up with the same level of complexity.
