Long story short, I agree with what you're saying, but you make it sound like it's really simple.
It's not.
Creating a single payroll solution to handle one company is very easy. Creating a single payroll solution to handle many companies is also quite doable.
But once you get to the scale of ADP, it's impossible to handle everyone elegantly. Between union contracts and non-standard practices, there are (what felt like) an infinite number of things a customer can throw at you, with one rule wackier than the next.
It's damn hard to write a generic payroll solution that handles most companies. Damn hard.