It's not that they didn't realise they were on the same frequency - it's that 2.4Ghz was really the only suitable unlicensed ISM band available. There's other

It's not that they didn't realise they were on the same frequency - it's that 2.4Ghz was really the only suitable unlicensed ISM band available. There's other devices there too - 2.4Ghz DECT phones for example when landlines were still a thing, 2.4Ghz video doorbells etc.