The incel sees no reason to have a platonic relationship with a girl. Everything he values in a friend, a guy can do much better (and without all that stress!), and that has been the pattern for all of his best friends so far. The only exposure to the opposite sex he gets is at work or through male friends; in both cases, he wouldn’t go out of his way to hang out with the girl in question but rather “puts up” with their presence to achieve an end (making a good impression at work or getting to the real star of the show - her male friends). Don’t get him wrong; he has some great funny moments talking to some of his coworkers, but all pleasure on his end was derived from the performance value of his shtick and the illusion of being a charismatic Chad - that is, not out of two way human connection, and definitely never platonically. You could consider it mental masturbation.
The incel had always assumed everyone thought like him up until about high school or so, but people left and right started treating women like equals, so he must have always been the one in the wrong. Why is he broken like this?