People want to board early because there’s never enough overhead bin space on a full flight
The size of some 'carry-on' baggage of late is getting comical - I've seen some as large as my checked baggage. And then some airlines allow an additional 'personal item' (laptop bag, handbag, etc) as well as the larger carry-on. Add to this an unwillingness to place anything under the seat in front (presumably to safeguard their limited legroom), and you're left struggling to find a place for your modest bag if you board last.
It's just selfishness, pure and simple, and it won't be stopped until the airlines enforce sensible limits.
My ideal boarding experience is to be the last one entering the plane. Which makes sense as I also try to cut my total time at the airport to a minimum. So far it has worked out.