Question on SSI - Serializable Snapshot Isolation

When using SSI, lets say we have two transactions T1 and T2 and there is a serialization conflict. Postgres knows when one or the other transaction is doomed to fail 

but will not raise serialization error until the transaction commits. This can cause a massive perf hit because the transactions could be long running. Why not raise the error early on when the conflict is detected to avoid wasting CPU and other resources? Can anyone explain this to me? Is this an area that could be improved upon? Thanks.

S.