Many simplifying financial concepts only apply to liquid markets. Market cap is one of them. The entire notion that a stock can have a price is another (if liquidity is low, you need to sweat the order book). Our modern financial system has opted to use these simplifications and limit casually-accessible trading to times of day (and days of week and options strikes and expirations etc) with enough liquidity to support the simplifications.

The alternative would be to gate everything behind the more sophisticated concepts and limit access to people who had demonstrated competence -- and that's exactly what we do for illiquid markets. It's not like exchange servers turn off at 4pm, if you want to step outside the high-liquidity kiddie pool after-hours and OTC trading exist. However, I am not scandalized that the kiddie pool is full of toys and you shouldn't be either.