The (US native, presumably) ATC staff speak worse English than the (presumably) non-native English speaker pilots, at least from all the ATC clips I've heard -- JFK and NY region being about the worst. Combining that with bad recordings and it's often difficult to understand, even as a native English speaker with a fair bit of familiarity with aviation (worked with pilots, although mostly rotary and military, working on a PPL)
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Non-native ATC staff speaks better English because they are non-native. They have to mentally "switch" to a different mode, so it is their second nature to stick to standard phraseology. The ATC staff outside NA talks to more non-native pilots than native ones, so even if they wanted to deviate from standard phrasing there's a decent chance the pilot wouldn't even understand them.
On the other hand, ATC staff in the US talks to mostly native pilots in their native language. It's really easy for them to use non-standard phrasing or slang, and most of the time it'll work out okay. With the exception of some key items like "cleared for takeoff" the FAA just doesn't really seem to care about it, so ATC will keep doing it.