That's because

now Psychotic, think.

"You're not funny" would be blunt and to the point. But maybe people wouldn't get it. Wit. Funny. EoFF. Hmm. EoFF is dumb.

OK. Uh.

"That's because you've never thought of a witty close message." Too vague? EoFF. Oh god damn EoFF I hate you.

No

no wait

I have it

ITALICISE witty. And maybe add a "Yeah?" at the start, to make it look like I type how people talk.

The final product:

Yeah? Well that's because you've never thought of a witty close message.