Why to clean a surface before painting

Every once in awhile, I get a request to letter a sign that the customer has painted themselves. The quality of their efforts varies. The pictures below are of yellow latex (water base) paint applied over a smooth oil base paint. The real problem is in the second picture; there's a layer of grime that was never washed off before applying the top coat. All the paint peeled off, the last picture is the biggest piece.
Seems to be a problem...
Dirt is the culprit
Coming off in sheets
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