I think the problem is that automatic seki detection is not as simple as it may seem, while it is needed to automatically exclude seki points from scoring under Japanese rules. So it is left to the players, assuming they know the potential pitfalls of the Japanese rules (as it would be when scoring an OTB game under Japanese rules).
Edit: KGS seems to have pretty good seki detection to correctly score games under Japanese rules (under the conditions that all neutral points are filled and dead stones are marked correctly by the players), but the creator of KGS (wms) didn’t seem to be 100% confident that his algorithm is always correct. See his 2005 challenge to come up with a seki that would be incorrectly scored on KGS: wms's Dame-Seki Challenge at Sensei's Library
If I understand correctly, in 2013 someone came up with a position that broke wms’s algorithm (winning a KGS+ membership with it):
I’m not claiming to understand why this position won the prize 