It's probably because you're taking the standard deduction.
To get the medical expense deduction, the sum of all your itemized deductions (which also includes things like donations and property taxes) must exceed your standard deduction amount.
You're only allowed to deduct medical expenses that go above and beyond 7.5% of your AGI.
For example, let’s say Pat is filing a single return and is itemizing this year. Their AGI is $35,000 and they had $4,000 in medical bills during 2021. Pat can only deduct $1,375 of these expenses ($4,000 minus 7.5% of her AGI = $4,000 minus $2,625 = $1,375).