Well, nobody speaks squirrel that I know of, but there are two theories,
The first is that squirrels are rodents and as such their teeth are always growing. So they have to have something to gnaw on to keep them in check. Lead is soft enough that they can chew on it while hard enough to wear down their teeth.
The second is that its sweet tasting. This is feasible. For those old enough to remember, almost all paint prior to the 60s was lead based. But small children would chew on this and get lead poisoning. They chewed it because it was sweet tasting. Now squirrels don’t get lead poisoning unless they get shot because they don’t swallow the lead, they just peel it off.
Just an interesting factoid about our little friends.