Even if this is a US only site, millions of Americans still have to convert as well. 2-3 hours may not seem to be a big deal but if they're planning for it, missing it by 1 hour could ruin someone's mood.
They sort of do, though I agree that it'd help a lot if the default schedule page had the multiple common time zones at least rather than just the one.
Anyway, it's started at this point (though it's only the google stadia done so far, with the 1st big one, EA, starting while I'm typing this) but the schedules for US/UK/AU times are below.
PT(/ET): https://www.gamespot.com/articles/e3-2019-schedule-all-press-conferences-dates-start/1100-6465962/ - this page also has PT/ET/BST/AET times but you have to pick them out of the paragraphs rather than a simple list.
BST: https://www.gamespot.com/articles/e3-2019-press-conference-schedule-uk-times-and-dat/1100-6467466/ - the 2 that say PT instead of BST are actually BST times still, they've just made an error in the text.