In addition to tijybba's excellent answer, it really comes down to pure numbers. It takes a few minutes to come up with an idea, it takes weeks, months, or sometimes even years to implement.
Give the average linux user 10 minutes to come up with 10 ideas and they will likely generate years and years of engineering work, which is why we can just write them down so if new developers come around they have something to work on.
Also a bunch of ideas do get solved, they might just not be marked off on Brainstorm or they're merely bugs or improvements that are just not implemented yet. Let's look at some examples from the most popular ideas from the last 6 months.
- Display downloading speed software center: just needs to be implemented, that's just a feature request.
- The shutdown screen is very outdated: Everyone knows that already, someone just needs to do the work.
- KDE native application should be marked in Software Center: just needs to be implemented.
- update-manager should choose automatically a mirror to update in case of failure: Actually the update-manager already supports the mirror:// method, it just hasn't been tested enough to be turned on by default.
- Magnet links should be supported in Ubuntu download page: Someone just needs to implement that.
- The family may use Ubuntu - which is basically "making users sucks", no argument from me there!
- Inexperienced users don't know when/how to safely unplug removable drives - that's a problem in every operating system.
- Be able to create a hot-spot in network-manager- We already do that!
- unnecessary complicated system boot menu - doesn't really matter because we hide the menu by default and only advanced users go in there anyway.
Now, some of these are good ideas, some are really just bug reports.
It's best to think as Brainstorm as a place for people to put ideas down so when a new developer wants to work on something they have a place to look at ideas, it's not a place to put a TODO list for existing Ubuntu developers mostly because they are a finite number and most of them are busy getting the OS out the door to users.
Also many of the pain points that users put down there are known problems. The guy who writes the shutdown dialog probably doesn't need to know it's old and crappy, he deals with it every day! Or my personal favorite: Speed up the file managers
It's just a matter of time to get things like that implemented, it's not as if the Nautilus developers intentionally made the file manager slow.
- TL;DR: Unlimited ideas, limited people to do the work.