Why all the "Re" in titles?
For the meaning of the word "Re:" in the title, depends on the Anime

(there seems to be a lot of explanation in the comment section of your answer).

But the reason on why there's an influx of it being used is the fact that it currently sells when used in a light novel, manga, or anime title.

It's like how in gaming after DayZ was out, there was an influx of zombie survival games.
or if you want an Anime comparison, it's like how after Sword Art Online, "Trapped in another world" anime popped up each season (starting the 'isekai' genre craze).

I'm not saying that everything with the word "Re:" in it is a cash grab, it's not; but it's more of a marketing strategy.

The earliest usage (that I can find) of this is in Re: Cutie Honey (2004) and later Re: Hamatora (2014) which is an OVA, and a second season respectively. The most impactful usage of "Re:" though is in Re:Zero.