This threads goes at length into that, no? Away from E3, Sony have the spotlight. They have a big enough brand to have all the eyes of the world on them whenever they choose to announce something. E3 was about getting everyone in one place to show stuff that they otherwise couldn't see, but that's no longer the case. So Sony can schedule things to their plan, picking date, venue, time, partners, and hold it whenever they want, meaning something like Apple or Samsung, "PS5 is launching in two months, in April because goonergaz recommended it," if they really wanted.
It's as much about not being in total control as anything. That's something Sony can do with PS as strong as it is but someone else would need to be at E3 to not be overlooked. SEGA or AMD would probably need to announce a new console at E3 rather than have their own event.
MS probably benefit more from being at E3 as the eyes of gamers who aren't Xbox fans are looking that way so they'll get better coverage with their messages (and lots and lots of social coverage as we discuss how confused we are over what the hell they're trying to say with those messages.
).