I think you may have a bit of a misunderstanding about what Meta is for, it is not a special place for advanced users to discuss but a help site (just like main) for Ask Ubuntu.
"new users can easily access Meta"
Sure they can, that is the point. Meta is where people can come and ask questions about the main site and new users are the demographic most in need of that help.
"Why posting meta requires only 5 reputation? Why not something like
25 or more"
The reason for the 5 rep requirement is to stop spam posts and it works close to 100%, I can't remember the last spam I saw here, it is not to restrict access to low rep users.
I don't think making it 25 or even 100 rep would bring the quality up, this site is low traffic (1 post a day even?) and as a result is well curated by its users, any bad or low quality posts would be removed quickly. Also rep means nothing, sometimes the best of us will ask bad and low quality questions, we are all just humans.
I would go as far as to say, in a spam free world, remove the restrictions and let anyone who needs help get it (that is the stack exchange way, anyone can ask). I could even argue that keeping the entry requirements low also cleans up the main site. People will just spam support questions all over main, if they can't post here. Then someone(s) has to review and migrate them all here anyway.
Anyway, that is just my 2 pence (what happened with that exchange rate?), I hope it helps you with a bit of understanding.