BRR explain why it's difficult. Even testing the grip changes the grip profile (via wear) but they do their best.

It's even more difficult with off-road tyres, as it's not really the rubber compound providing the grip.

Even that is not realistic as in contradiction with 4 wheels setup most people on pavement don't nearly reach the limit of grip on 2 wheels. The reward/risk ratio is not high enough. You'd need a bot for that on a skidpad test.

Also on 2 wheels going fast around corners doesn't only involve grip but confidence. This is partly personal/psychological and partly based on feedback from the tire that comes on how the carcass of the tire deform while you are leaning the bike. It could be that a tire with more grip doesn't give you as much confidence as a tire with less grip on which you are closing in much closer to the limit.

Off the road the grip is so dependent on surface that it is even more difficult. There are tire for dry, mud or mixed surfaces and what you will encounter on the trail might be somewhere in between all of these.