When spoken should’ve can sound a lot like should of or shoulda depending on the dialect. Most native speakers don’t really think about gammer rules when writing informally, they just write how they speak.
shoulda makes vastly more sense, and is precisely the kind of thing that tends to become slang and eventually just the standard spelling, like “dunno” or “gotcha”.
“should of” just feels like people know it should be two words, but don’t know how it’s spelled, and instead of just spelling it phonetically they make something half-phonetic up.
When spoken should’ve can sound a lot like should of or shoulda depending on the dialect. Most native speakers don’t really think about gammer rules when writing informally, they just write how they speak.
shoulda makes vastly more sense, and is precisely the kind of thing that tends to become slang and eventually just the standard spelling, like “dunno” or “gotcha”.
“should of” just feels like people know it should be two words, but don’t know how it’s spelled, and instead of just spelling it phonetically they make something half-phonetic up.