You can select all the samples with shift-click or ctrl/cmd-a (middleclick to get keyboard focus on the samples frame first!) and then just click “autoseek” once.
Autoseek is not enabled by default because having autoseek turned on by default on all samples is very cpu intensive,this has been explained before.
Conner wrote a code-snippet in Lua that does enable autoseek for every available sample in your current song, but the current code needs adjustments for it to work in Renoise 3.0 (API 4)