Likely because there are more similar words using a circumflex:

• bâté
• gâté
• hâté
• pâté
• tâté

than not:

• daté
• maté

The eye, more accustomed to see a circumflex accent in four-letter words ending in "-ate" affects the writing of the word "raté".

The influence of the spelling of one word to another is a common cause of mistakes. For example, barrique sometimes lead people to write baril with two R’s, barril. The word artichaut (artichoke) is often written artichaud by the influence of chaud (hot).

Note that a circumflex accent changes the pronunciation of the A in francophone Belgium and Switzerland but doesn’t make a strong difference or any difference at all in many parts of France except eastern regions.

See this Mathieu Avanzi’s page

I guess people living in regions where there is no distinction are more likely to miswrite raté.