Led Zeppelin was influenced by musical traditions that placed an emphasis on playing with intonation and rhythm as a technique for conveying emotion and creating musical tension. Producers back then would have known that, and known knot to mess with it even if they did have the tools back then.

But also, a perfected, highly produced sound is part of the aesthetic values of modern popular music. So producers who do it now aren’t necessarily wrong, either. They’re just following taste. If people start loving a loosey goosey White Stripes sound again, producers will go back to leaving it in.