Former lightship "CALSHOT SPIT" at Southampton - 09.03.2019

Interesting detail: This lightship was replaced by a "navigotional mark" still called "CALSHOT SPIT" - and it's still marked in the charts as a lightship... (why?)
You can see the actual "CALSHOT SPIT" in the foreground of this photo:

https://www.marinetraffic.com/en/photos/picture/ships/3415771/9437919/247293300

Unfortunately the original photo on shipspotting.com has been deleted by the admins because of a ridiculous reason...

You see, that "CALSHOT SPIT" in the foreground is still shaped correctly like an anchoring lightship... nice :-)
But why not mark and install a simple bouy? A kind of a nautical tradition?