It keeps their narrative that it was stolen alive...
My own memory is foggy...but I remember that really only staunch MAGA supporters thought Trump won that 2020 debate. Many of my own GOP friends, that were fairly ardent Trump supporters, admitted Biden beat him...most quickly pivoting to blaming the moderators and/or the questions.
Biden was declining at the time, but he was functional enough to beat Trump in that debate. And more than enough people were o.k. enough with this mental and physical state, to vote Trump out of office. It wasn't stolen. Hundreds of courts, many of them led by GOP appointed judges, have clearly and definitively said as much.
But, when you lose, and you're weak AF, you look for excuses rather than solutions.