I can only speak from Capcom's perspective, but it certainly would be deflating for our dev folks who have worked years on it before reveal, and it would impact our marketing plan to some degree.
Leaks for smaller titles such as port projects that tend to leak via ratings agency are relatively benign as we can't always control it. We expect some of it to happen.
But leaks for major titles that are bound for reveal during a first party press conference would really sting. For any press conference material, we bring our A game with real gameplay footage and cutscenes and want it to be a genuine surprise for everyone.
I'm sure many folks here understand, but there's lots of coordination that happen behind the scenes between the dev team, marketing, sales, PR, social, and last but not the least, first party.
From conceptualizing and storyboarding the trailer suitable for the show to coordinating specific announcement times and delivering assets so they also show up on first party channels right after announcement. Things of that sort that require months of planning, communication and coordination amongst stakeholders. ...and it leaks out of the blue. It just poo-poo's everyone's hard work who have been preparing for the reveal.
Capcom, for the most part, have been fortunate. Looking back at RE7, MHW, RE2, and DMC5 in recent memory, most of them got through relatively unscathed with just a couple that leaked hours before the official reveal.
At the end of the day, E3 is a 'show' in and of itself — an annual spectacle of showmanship and an opportunity to excite and delight fans and the media.
We get it. Leaks are fun and exciting as it stokes fire and drives conversation before the official reveal. It's just that early leaks deflate some of that excitement and eliminates the element of surprise.