Event Mod Question

I wouldnt say im "new" to warframe, been playing for a while, but there are still things im not entirely sure about.

Im a pretty casual player, so Im certainly not geared up much at all. No primed mods, not many event mods etc. The most I can boast is a handful of prime equipment and frames.

Now, once an event comes it brings mods a plenty, sure, but given my casual nature playing the game, generally my opportunity to attain such mods is slim. Then, after the event ends, all of the mods seem to vanish into the ether.

My question is do these mods ever get into the regular circulation? There dont seem to be many cool mods to go after any more now that Ive got most of the ones from the vaults, and the nightmare missions. Cant help but think my "growth" as a mod collector is being stunted by the fact that I cant play all the time.

Maybe its just me. Can someone give me a little insight?