Why do planes not give off a tracking signal ?

[chaparral]

With the amount of great technology we have these days why are planes not fitted with something that gives off a signal of there location when they go down

I really no understand how something like this is not available or fitted to all planes these days.

This news here is what got me thinking of this
https://guernseypress.com/news/2019/01/25/missing-aircraft-search-is-brought-to-an-end/

 

[chaparral]

Black box transponder?

Not sure if they even work after a plane has gone down as there been quiet a few planes gone missing (including airliners)

Including this well know one
https://forums.overclockers.co.uk/threads/contact-lost-with-malaysia-airlines-plane.18585604/

 

[chaparral]

Seems an oversight as ships have AIS in order to track them https://www.marinetraffic.com/en/ais/home/centerx:-0.5/centery:50.6/zoom:4

But can they still pickup the signal from it if the boat sunk and was at the bottom of the ocean ?
Or does being under water completely block the signal