Why does USB only use 2 lines for RX, TX instead of multiple data lines?
It would be faster indeed if instead of one line you'd feed multiple lines at the same symbol clock.

But, USB's primary and foremost goal  is to provide easy, serial (hence the S in USB) interfacing between low-cost devices (hence the U in USB) with low-cost, lightweight cabling.

So, that's why USB doesn't do parallel data lines: It's simply not the niche it's supposed to fill.

Also, don't neglect that having multiple high-speed parallel lanes requires the transceiver system to introduce a relative high amount of effort to compensate different skews on different lines, which at high rates are inevitable.

It's often become cheaper to make something work twice as fast than building two of the slower variant, unless you're really directly talking to hardware that is in its raw principle bit-parallel (e.g. DDR memory chips).