Toddlers live entirely in the 'now'. When you say 'nap time', they hear 'stop having fun'. To them, this feels like an unfair loss of control. A 'bridge' activity helps them transition their focus without the power struggle.
The 'Bye-Bye' Routine
Try having your toddler say 'goodnight' to their toys. This externalizes the transition, making it about the toys needing rest too. By the time the toys are put away, the toddler has mentally shifted toward the idea of sleep themselves.