TLDR: capital efficiency, as well as decentralization.
How many chains is FRAX on?
FRAX the stablecoin is on over 20 different chains. Ethereum, Fraxtal, and Arbitrum are the largest. FXS, FPI, frxETH, sfrxETH, and sFRAX are also on many of these chains.
Why does the Frax ecosystem have so many tokens and sub-projects?
The purpose of sub-projects like Fraxlend and Fraxswap is to ultimately generate utility for FRAX ecosystem tokens (like FXS, sFRAX, frxETH/sfrxETH) and stabilize / collateralize the FRAX peg. The goal was never to compete with larger competitors (e.g. Aave for Fraxlend, Uniswap for Fraxswap).