There's not more lanes available. The generations are getting faster just by increasing the transfer clock rate, up to PCIe 5, and in PCIe 6 by increasing the number of bits per transfer. The way they doubled the speed every generation was pretty basic: the timing tolerances were chopped in half every time. The allowable clock phase noise in PCIe 4 is 200x less than in PCIe 1. The miracle of progress, etc.

That miracle is somewhat over. They're not going to be able to drive phase noise down below 1 femtosecond, so 6.0 changes tactics. They are now using a fancy encoding on the wire to double the number of bits per symbol. Eventually, it will look more like wifi-over-copper than like PCI. Ethernet faster than 1gbps has the same trend, for whatever it's worth.