A lot of data throughput and buffer just for ingesting and distributing the live streams themselves, technical and business administration to keep things running, moderation to ensure compliance with content laws and data protection regulation, and then there's still all the other fancy features major platforms offer if you want to compete for users.

Multiple resolution options with server-side rescaling for users with slower connections? Graphics computing power.
Store past broadcasts? Massive amounts of data storage capacity.
Social features? Even more moderation.

And we haven't even touched on the monetary issue of "How do you pay for all that?" and all its attached complexity. You could be running the nicest platform in the world, but without any funding, it won't run very long.