Why are relays still used in electric ovens?
Advantages of relays over triacs:

Very little voltage drop when on.  This means they don't dissipate much power.  For high power devices, the cost of dealing with the heat often outweighs the cost of the component that dissipates the heat.

Good isolation.  The relay coil is inherently electrically separated from the relay switch.  Making that isolation withstand normal power line voltages is pretty easy and cheap.

Able to withstand high temperatures better than semiconductors.  Silicon stops being a semiconductor at around 150 &deg;C.  It's not too hard to make relays that can withstand substantially more.  That can be quite useful when in a device that is intended to get hot.

Better input noise immunity.  Stray capacitive coupling even from nearby power spikes, RF pickup, and the like aren't going to trip a relay.