They just don't 'need' to, and I think that's the problem.
None of these spaces they rent their massage shops out of have high expenses to maintain. They are avoiding taxes as I assume none of them are being honest with their earnings. And even the laziest shops still probably get more men visiting than you think.
Sure they'd probably make more money if they put in effort and had the drive to grow their customer base, but if they had that kind of high work ethic, they probably wouldn't be working at a rub n tug.
They probably get by just fine doing the absolute minimum.