WPSE point of view
The majority of plugins is outdated.
Plugin dev point of view
You get no profit (unless you take non-calm, pushing and bold users and call them "worth it"), so most plugins stop their dev cycle, stay in the repo 1) and get slowely outdated 2).
1) Official repo has no clean-up crew, so they simply stay.
2) Some get outdated fast (Ex. Image Uploader > Plupload).
Users don't take a look at the date/version, the Q was asked and they simply download (the yet - 8 years later - available) plugin and use it.
Then we have to clean up the mess and have a few new Qs:
»I got a blank page! Help!« → »How do I activate debug?«
»I got dozens of errors! Help!« → »Turn off all plugins, switch to blablabla…«
»My Plugin isn't working« → »You can fix it with this or that" → »Where do I place this code?" → »Well… sigh … you're better off searching for a new plugin.«
»I need a Plugin for something incredibly kool that can do [insert task that you never imagined]" → »Try this or that.«
BACK TO N°1… ✓ *)
*) Which means we just double the users that a) backtrace it and try to fix the longely outdated plugin, which brings up more annoying Qs and b) more users that have problems when the new plugin recommendation is outdated in some month. The snake bites her tail…