If the cable can carry that much current safely then why isn't it rated at
30-40A?

When they rate a cable for 27A, they are also saying that it can really take
twice as much, but they've left a whole load of headroom to allow for short
term (i.e. 10s of minutes) of transient loads (such as
toasters/kettles/washing machines etc).

There's a reason that MCBs are only guaranteed to trip quickly at 5 times
nominal current. It is because the cables are man enough to take 5 times
current for a short period and there's no need to design a system that cuts
out just because with a heavily loaded system you've put the kettle on for 2
minutes, or a motor is winding up.

Christian.