Because people are throwing a lot money into the rent black hole and even a little equity is an improvement over that and mortgage interest tax deduction can be huge
You can deduct home mortgage interest on the first $750,000 ($375,000 if married filing separately) of indebtedness. However, higher limitations ($1 million ($500,000 if married filing separately)) apply if you are deducting mortgage interest from indebtedness incurred before December 16, 2017.”
It used to matter more, before the Trump tax plan significantly increased the standard deduction. The mortgage deduction is bad tax policy but it's hard to get rid of.