I understand that the X-Men and the Avengers are both based on Marvel comics series and they seem to often share adventures in the comics (for example Wolverine and the Avengers appear together both in this picture from the Marvel website). Marvel founder Stan Lee also tends to appear in both film series.
But I haven't seen any of them run into each other. Aren't they living on the same planet? Do they ever meet? If not, why not?
It's an issue of who owns the rights to what characters. For example, Sony owns Spiderman. Disney had to license the character rights from them in 2015 to include him in the filming of Captain America: Civil War and Spiderman: Homecoming.
Similar situation with X-Men, as Fox Studios owns the rights to them. However, if Disney can win the acquisition deal of that portion of Fox against Comcast, they no longer have to worry about licensing the X-Men any longer, and we could see an introduction of them into the MCU.
Check here: http://www.businessinsider.com/disney-fox-deal-means-x-men-deadpool-fantastic-four-join-marvel-cinematic-universe-2017-12 .
As well as here: https://www.theverge.com/2018/6/15/17461908/disney-comcast-21st-century-fox-acquisition-franchises-companies
