well - it's also possible to scramble calls on the PSTN network, but criminals often don't avail themselves of this ability. Which is presumably also the case on the internet. Hence a 'basic' level of IP wiretapping would still be useful to crime investigators.
They already have the basic. What they are terrified of is precisely that technological change and decentralisation has compromised their ability to do "non-basic" wiretapping, and want to roll back those gains.
Also, scrambling calls on the PSTN requires somewhat non-ubiquitous - if not necessarily any longer prohibitively expensive - acoustic coupling equipment. By comparison, PGP, VPNs, and TLS are much easier for an average person to use. To some extent, everyone uses these things whether they consciously sought to or not, at least in the case of TLS certainly.
