Why can’t a formula number field be used in the rollup summary field salesforce?

A formula number field be used in rollup summary field salesforce because - Formula fields can only be used in a rollup summary if they are idempotent (in other words, if they return the same value every time when the record is not edited). This means you can't use cross-object formulas, or formulas that depend on TODAY() or NOW(). This is noted in the documentation: If a roll-up summary field doesn’t contain cross-object field references or functions that derive values on the fly, such as NOW or TODAY, it can calculate the values of formula fields.