I think there's a difference between an incel and someone who's simply lacking success in relationships. For a start thanks to coronavirus and lockdowns there's a
lot of otherwise totally well-adjusted people who would be put unfairly into that category, and secondly I'd categorise incels as people who specifically make their lack of romantic success part of their emotional identity. This is unsurprisingly a bit of a self-fulfilling prophecy as people who do that tend to have pretty insufferable personalities as a result. Instead of working to improve their attractiveness they tend to fall into this nihilistic belief system where all the blame is placed on external factors (usually women) and there's nothing they can do personally to improve their lot in life.
Everyone has dry spells, sometimes even quite extended ones. That's perfectly normal until you start adopting this as part of your personality and taking this frustration out on the world around you. Incels who self-identify as such often have a somewhat organised and deeply revolting ideology too, whereas "people who've not got laid in a while" is a set that nearly everyone has been part of at one point or another so isn't a marker of anything specific.