Can't agree at all about a sight glass. The lowest point in nearly every brake system is the brake calipers themselves. Wheels going to make it difficult anyways on a car. Not to mention the risk of a rock striking and damaging rigid glass. Or the old interface that's sealing the glass to the calipers wearing out and causing a leak.

It's not worth the expense to manufacture and it's not worth the risk. Not when a litre of DOT4 is $20 and that only needs replacing every 3 years at an aggressive schedule for passenger cars.

Larger vehicles you can purchase a brake fluid tester, but most of the really large ones I know use air brakes anyways.