Besides the "we already have it everywhere" bit, the big advantage of analog AM over UHF/VHF is that it degrades fairly gracefully. As you're getting too far away the signal gets harder to pick out from the noise, but it's not an all-or-nothing digital signal.
That doesn't matter much over Washington DC, but when you get out towards the western half of the country the transmitters are a lot more widely spread out. There's mountains in the way. There's limitations to how low you can be and still be reachable, which sometimes has to be balanced against how high a GA plane can comfortably fly, or oxygen requirements for the occupants.
A better sounding "modern" system is generally going to be worse at handling those marginal situations, which would probably require building a lot more radio outposts in fairly remote areas to compensate.
But the big problem with requiring anything new is getting it into the existing fleet of thousands of decades old certified aircraft. You need a new radio stack. You probably need new antennas. Changing anything on certified aircraft needs tons of paperwork and things like Supplemental Type Certificates for each individual model of aircraft that make it cost 5-50x what you'd think it should cost. Military aircraft are probably 10x worse beyond that.
A handheld COM radio is maybe $200 from Sporty's. Take basically the same thing but package it as a basic Garmin COM radio (GTR 205) and it's now $2,300. If you want a NAV radio in it too (GNC 215) now it's $5,400. Add GPS and ADSB-Out (GNX 375) and now you're at $9,000. You can buy an entire currently airworthy (really old) plane for maybe $30,000.
For some uses you don't even have to have any radio or transponder/ADSB installed on your aircraft. Some aircraft don't even have an electrical power system to run one. Granted they're not allowed in the middle of Washington DC, but still trying to require the entire fleet gets fancy new digital radios would be a monumental challenge and fantastically expensive.
There are some existing better ways to communicate. Larger planes usually have CPDLC (Controller Pilot Data Link Communications), which is basically text messaging from ATC to the plane avionics. That's how they receive their IFR clearance at major airports. In an airliner you're not generally reading it out over the radio and scribbling out your "CRAFT" acronym on a scratchpad like you see on YouTube. (Even your 4-seat steam gauge Cessna can do this via PDC (Pre-Departure Clearance) at supported airports with something like ForeFlight on an iPad).
CPDLC can also be used in the air to communicate less time-sensitive things like altitude changes and reroutes, talking to the operations department of the airline, etc. There's no reason you couldn't put this in smaller GA planes and towers, other than cost.
Airliners that do transatlantic and -pacific routes also have satellite communications instead of using old-school HF radios (which are even worse than VHF).