and the british electorate could have used its power in controlling the levers of one of the most pampered and privileged members of the eu to keep laws friendly to british business in the same way they've done it since thatcher, but apparently that wasn't good enough

This was already the case before Brexit, via Parliamentary sovereignty.

If what you said was actually true in practice, then why would people be mad that they were leaving?

Various reasons, e.g. losing the perceived economic benefits of a free trade zone with the rest of the EU countries.

EDIT:

I can't reply to the comment below for some reason, so here's what I was going to say:

Much of the EU's unpopularity amongst the Brexit-supporting section of the UK's population comes from it repeatedly being used as a convenient scapegoat for domestic political issues, and the country's most-read newspapers printing decades of nonsense ("EU bans yogurt and cheese from school dinners", "EU bans straight and bendy bananas", etc.) that were taken as fact about what a broken institution it is.

Brexit was really just the end result of a spectacularly successful sustained propaganda campaign.

If that's all the eu were I'm sure it would be smashingly popular.

Losing access to the schengen zone travel and trade wise is a big deal. IIRC EU market is largest single market, or was. 'The Troubles' was really devastating and people rightfully fear going back.