Because it's part of the whole picture.
Engadget published a break down of the cost of an E3 booth back in 2013. An indie studio ended up spending $100.000 on the entire show:
Now put those numbers in the context of a company like Sony, who use to have one of the largest booths at E3.
Sony were easily spending seven figures on their showfloor presence alone. That doesn't even include staff, food, hotel accommodations or travel expenses ...
That's a lot of money for 70.000 attendees (including a few thousand fans who paid $149-249 for a three day ticket). At the same time you have public events like Gamescom, Paris Games Week, Tokyo Game Show and Brazil Game Show that attract massive crowds offering a much higher return on investment.
The vast majority of those attending E3 are industry professionals, journalists, content creators and influencers. There are much more cost effective ways to interract with them (or reach the audiences they represent).
