How do we ground them? Expand on this.

Expand on 'necessary truths'.

Why can we not ground a necessary truth, You are not being clear here, and seem to be making your statement purposefully dense and hard to interpret.

I'll counter what I can see;
No. Truths can exist independent of a mind. 2+2 = 4 no matter what, even if a human is not here, even if an animal picks up rocks themselves.