because it _does_ provide a number of benefits (potentially fewer initial round-trips, more dynamic routing control by using UDP instead of TCP, etc), and is a userspace softare implementation compared with a hardware-accelerated option.
QUIC getting hardware acceleration should close this gap, and keep all the benefits. But a kernel (software) implementation is basically necessary before it can be properly hardware-accelerated in future hardware (is my current understanding)
To clarify, the userspace implementation is not a benefit, it's just that you can't have a brand new protocol dropped into a trillion dollars of existing hardware overnight, you have to do userspace first as PoC
It does save 2 round-trips during connection compared to TLS-over-TCP, if Wikipedia's diagram is accurate: https://en.wikipedia.org/wiki/QUIC#Characteristics That is a decent latency win on every single connection, and with 0-RTT you can go further, but 0-RTT is stateful and hard to deploy and I expect it will see very little use.