Why are polymerization reactions sometimes called dehydration synthesis reactions?
This only applies to specific types of polymerisations.
Polymerisation by step growth involving condensation reactions where the molecular group eliminated is water are sometimes referred to as "dehydration synthesis" reactions.
For example, the formation of polyesters via the reaction of polyols and dibasic acids - esterification reactions take place by which the various monomer molecules link together, and as water is removed in the process, they can be considered as "dehydration" reactions.
This does not apply to chain growth (radical) polymerisation, nor does it apply to condensation (step growth) polymerisation where the group removed is something other than water.