Why can't Jews use ovens during holiday?

Answer 1

There are a number of specific requirements that Jews have for behavior during holidays. There are occasions when ovens can be used. However, on Shabbat and Full Holidays, Cooking with heat is a prohibited practice. It violates the prohibition of Bishul (Cooking) which was one of the labors required to make bread (which was one of the four main categories of religious holiday prohibitions.

However, slow heat from an extant fire under something prepared prior to the holiday is permitted to keep it warm since the "cooking" already occurred.

Answer 2

because we can use fire/electricity