ERRETHEE

Matthew 5:21 includes an aorist passive from of said.

Rather than just use the aorist it uses ἐρρήθη

“You have heard that it was said (by) the ancients,”

Not merely it was or is said.

But how can ἐρρήθη be the aorist passive form of λεγω?

Originally judging from Epic and Ionic forms the verb to say speak or ask was * ἔρομαι with a stem of * ero cognate with Latin (v)erbum.

That explains partly the er but why erre. Greek uses reduplication and not just in the perfect.

Greek also uses metathesis.

Plus sometimes the present tense of a verb gets replaced by another verb.

Finally the reduplication and reversal of the stem also reduces possible confusion with similar verbs with an ree stem in the aorist like rhiptoo and p(h)eoo. And r(h)gnumi.

Well those are some of the reasons. Its just proof of Heraclitus' famous saying: 

Panta rheei Everything flows! Maybe?