A well landscaped yard does add value. "Curb appeal" is the first impression on a home and it does affect price.
It takes away value for a buyer who doesn't want a pool.
I have had buyers tell me not to show them homes with pools, but have never had a buyer tell me to ONLY show them homes WITH pools.
Appraisers will add some small amount to the assessment of the home with a pool, but nothing even close to the cost of adding the pool.
Depending on the lushness and design of the backyard landscaping, some increased value may also be added.
Perhaps the homes you are looking at are simply overpriced - pool or no pool!