Because beginners are taught to start with 1.e4 as it presents a more "open" game with more tactics.
Why does everyone play e4 as white?

e4 is just a great move. You'll reach exciting positions full of tactics and fun. And fun is what chess on the internet is all about.

Because they like it better. e4 is a more popular move than d4 at almost all levels.
e4 being a better move for beginners is a myth. Whatever gets your pieces out is good, and e4 does that. So does d4
When I was 1100 I played d4, too.

1.e4 is a good move played by masters. if you want to get beginners out of their comfort zone, play and revise thoroughly the sicilian.
make sure you revise the sidelines as you'll see that a lot against people who don't know what they're doing.

Because Fischer said e4 is Best by test.
D4 is positional with tendency for draws hence used by players like Karpov/Kasparov.

At my rating level nearly all my opponents play e4 as white.
I play e4 as white maybe once every 20 matches.
I play d4 as white 90% of the time.
The argument between e4? and d4? is irrelevant, as everyone knows Nh3! is a forced win for white.
At my rating level nearly all my opponents play e4 as white.
I play e4 as white maybe once every 20 matches.
I play d4 as white 90% of the time.