Why do adults get baptized in a pool?

Answer 1

yes they can

Answer 2

Infant baptism was introduced by popular demand in response to the huge infant mortality during the epidemics that swept Europe after the fall of the Roman Empire. Before that, all baptism was in pools. The concern was that all the children who died as infants would go to Hell. Adult baptism using a pool in which a person immerses as part of a conversion ritual is a tradition Christians inherited from Judaism. Conversion to Judaism and various Jewish purification rituals all involve dunking in a pool called a mikvah. If you visit archaeologica digs of first-millennium Christian churches, you can see the baptismal founts that were used by the Church in that era. They did not differ significantly from Jewish mikvas from the same era. (The dig under the cathedral in Geneva Switzerland is an excellent example.)