So why didn’t he give it to them while they were alive?
Let us say the current gift tax limits apply, thus you, as an individual can give $15k/year to any number of people without penalty to you or the recipient. Double the amount if married even if filing a MFJ tax return. Go over that amount and you, the giver, should start filing a gift tax return which MAY mean paying taxes on the gift.
IRS Form 709 gives details but basically any gift in a single tax year is based at 'Fair Market Value' and is cumulative over the lifetime of the giver to the recipient. CURRENT lifetime tax-free limit is $11+ million but in the recent past it was as low as a single million. Also note that earlier years had lower exempt reporting amounts as low as $10k.
In short and not knowing the circumstances of the inheritance, the parent could have been land-rich and cash poor. Giving away the land was probably impractical before the death and expensive afterwards.
NOTE: This is NOT Tax Advice. While I am an Enrolled Agent (EA), there is always something that makes an individual's tax filing unique and what I have written above is very generic. Do your OWN study and, if needed, consult a tax professional!