If their baby(ies) were from Alice's ova and Frank's sperm, why did they have to grow in Phoebe's womb (her oven, totally their bun, as Phoebe describes it)? I know out of universe it was to explain Lisa Kudrow's eventual baby-bump- was there an in-universe reason?
In universe -
Alice(44) was Frank's(18) home economics teacher in high school. They both start dating in season 3. They get engaged and Alice is introduced to the gang in the episode "The one with the Hypnosis Tape". They decide they want to get married and have babies.
In "the one with Phoebe's uterus", they get married and they ask a favour of Phoebe to carry their embryo/child. Since at an age of 44 the rates of conceiving babies comparatively becomes difficult, they ask Phoebe to carry their baby for them. Phoebe accepts to be their surrogate.
Coming to the show the writers probably thought that would be good way to utilise Lisa Kudrow's real pregnancy