Why are there not many rivers that empty into the pacific ocean from the us?

Lots of rivers flow into the Pacific Ocean.

Not too many rivers flow into the Pacific Ocean from the United States, because we have mountain ranges that coalesce many streams / rivers into a fewer number of rivers than can cut through / around the ranges. As compared to the Atlantic Ocean and the various "Gulfs of".

Additionally, much of the area of the United States that drains to the Pacific Ocean is arrid, due in large part to the aforesaid mountains.

Answer

It is bordered by mountains.