You "can" do all these things on BTC. Almost nobody does. All of Tether & USDC usage takes place on other blockchains. The lightning network is notoriously tricky to use and isn't trustless. A diversity of assets enables things like PAXG, an ERC-20 token backed fully by real-world gold, as well as things like tokenized stocks, bonds, and t-bills, the last already existing, and the former surely coming in the near future.

Additionally BTC isn't "over 50% of the market", it's over 50% of the market capitalization, which is a huge difference. BTC doesn't have the raw TX speed to make up 50% of actual market activity.