Trump is so sure that the election is rigged because he rigged it for himself. But it wasn't enough.

He is simply delusional. He was so fixed onto rigging the election that, whatever the result, he would know/believe that it was indeed rigged.

We know he tried to cripple mail-in voting by using his power over the USPS. We know he tried to overturn the results after they came in, with numerous frivolous lawsuits. He has no integrity and will do anything to win.

Polls showed a 69% chance that Biden would win Florida, yet Trump won Florida by 3%. I wouldn't be at all surprised to find out that Florida voting machines changed Biden votes into Trump votes, and likewise in other states, but it wasn't enough to overcome his incredible unpopularity.