They do the maths and they come out with chances of it happenning so low, that it doesn't justify the installing of the fourth system.
There are plenty of planes with 4 hydraulic systems.
A300 and DC10 have 3 redundant hydraulics. B747 and L1011 have 4. No one seems to have bothered with 5.
The JAL B747 which had a rear bulkhead explosion lost all 4 redundant hydraulic systems, just like the DC10 lost all 3 to tail engine explosion and the DHL A300 lost all 3 to a rocket hit in wingtip.
It follows that multiple common-cause failures are not an unlikely event - has happened 3 times already - you could be the fourth.
A320 has 3 hydraulic systems - like A300. But unlike A300 and A310, the A320 has glass cockpit, fly by wire and sidesticks.
If an A320 were to lose one wingtip and all three hydraulics just like the A300 did - how would the cockpit controls react?