Why don't relays incorporate flyback diodes?

Warning: this may be an extremely naive question (if so, please enlighten me).

Many applications of relays require a flyback diode to protect against inductive voltage. I'm unable to find any relay that incorporates a flyback diode.

Since it's such a common need, why don't relays include a flyback diode inside the relay package? Are there just too many factors to consider, making it hard to guess the circuit's need?

Answer

There is simple answer to this question - there are many flyback schematics and the reverse diode is the simplest one. Although it has one big disadvantage - it makes the relay to switch off very slow.

This way, sometimes other schematics are to be used. There are several examples:

^{simulate this circuit – Schematic created using CircuitLab}