“Can you pass me the bag of plastic blocks?”

Not

“Can you pass me the bag of Legos”

If you wanted a box of adhesive bandages. You’d say “Can you pass me the box of band-aids?” or a tissue, “Can you pass me a kleenex?”

Companies do not want their brand to become generic trademarks.

It’s not because Lego is already pural

“I have many Lego blocks”

Not

“I have many Lego”