Tbf, in terms of starting salaries, most maths/physics ECTs can negotiate starting on M2, if they want to- not in every school, but in a lot. Obviously they can't negotiate higher pay than UPS3, but for maths at least, there are also retention payments throughout their early career as well.
It doesn't seem to make much difference to whether they leave. Also, as others have said, schools now often struggle to recruit for everything from MFL to DT to Geography- so you'd just end up paying nearly everyone the higher rate. And what happens when people teach outside of specialism, as well?
Anecdotally, for physics teachers, the biggest issue isn't pay, but being asked to teach outside their subject area- especially biology. A lot of them hate it, more so than biology teachers being asked to cover physics. But it's often necessary at KS3 at least to make timetables work, now- and teachers of other sciences aren't so common that schools can afford to screw them with the timetabling- even if they theoretically could.
Another issue is the pressure of having to take on a lot of exam classes early career- often a physics ECT will be teaching multiple Y11 classes + Y12 and 13 from the start- and it's a lot to deal with in terms of both pressure and workload.
I think, in a lot of cases, you'd be better off giving science teachers more PPA to take into account the fact we're often planning and teaching 3 subjects, rather than necessarily more pay. I'm sure the pay is a factor in people not choosing teaching to start with, but workload is a much bigger factor in causing people to leave.