Because the tube-screamer approach works better, simply put. There's no way to get that sound with a "pickup with the tubescreamer aspect built in" without it being terrible.
It's part of an overall larger debate when it comes to recording, signal flow, and stuff like that about a difference in approach philosophy between "capture everything and filter stuff out later" and "capture only what you need".
Obviously most people go for an approach that is somewhere in the middle of that spectrum, but going with pickups meant to mimic the qualities of a tubescreamer is just too restrictive. It has to be a separate aspect of the signal chain to work well.
Of course, there are pickups and amps and pickup/amp/guitar combinations that don't require a tubescreamer at all. Even for extreme metal.
But the whole "designing a pickup to mimic the aspects of a tubescreamer right out of the box" thing is a terrible idea.
Tube Screamers also compress your signal quite a bit, and most people prefer to have the signal out of the guitar more uncompressed and then apply compression to it to taste at the inumerable number of stages where compression is involved between when your signal leaves the guitar 1/4th out and it hits peoples' ears.
Also, cutting the low end out at that early of a stage completely kills the ability to do thrashy style palm mutes. If you want like, the style of palm muting Metallica popularized (which is obviously the only style of palm muting out there for the trve), you need the low end for the feel, even if you are getting rid of it later.
High passing with a nice usable shelf at ~70-90HZ on a recorded signal of a guitar near standard tuning is a fantastic idea that is pretty bog standard....
But it's definitely not the same thing as high passing the signal at that same place the second it leaves the guitar.
Just because you don't want to hear those frequencies at the end doesn't mean you don't want them hitting your amp. Or hitting a pedal, or whatever.
FWIW, Tube Screamers also aren't a hard high-pass either. They have mix/balance/level knobs that most people keep around noon/50%. They aren't actually used as a strict high pass cutting all those frequencies out.