One pair of pliers isn't enough, you need 2 pairs, because the valve is twisting as you apply force.
You need one pair on the valve & the other to turn the cap.
i bough some cheap aluminium type years ago the stuck like superglue to my wheels even pliers had a job to shift them. They seem to corrode themselves to the valves
Dust caps shouldn’t be on long enough to get stuck on, if you are removing them to check tyre pressures regularly, (about once a month) as recommended, as they do lose pressure over time.
Correct pressures are important for safety, legality and fuel consumption.
I use plastic caps, and when I had a problem attaching a tyre inflator, I cleaned the whole length of the threads carefully with a little WD40 on a cloth.