Because overperforming expectations doesn’t denote better play.
Take Anthony Martial, nearly 7 goals above expectation last year. That’s broadly unsustainable, he was likely fortunate with multiple goals, and he’s regressed this season to almost two goals below expectation in spite of very similar xG per shot numbers.
Same goes for Mason Greenwood, 6.4 goals above expected last year, down to -1.7 against expectation this season.
You can call that form, but the theory behind the stat is that even elite players won’t outperform expectations forever. The model is meant to continue to adjust for it, though it regularly lags behind.