Is there is already such restriction? If no, why not restrict questions to some max length to avoid too long questions?
Such questions are usually difficult to read and too vague.
Is there is already such restriction?
Yes there is already a limit of character length on posts. The maximum length of any post (either question or answer) is 30,000 in Stack Overflow.
Some questions need to be long in order to fully explain the issue. It is hard to imagine a hard limit that would work for all Stack Exchange sites (or even within a site).
For example - if a question on Stack Overflow requires a code listing, an explanation of what it does, what the OP tried and hasn't worked, sample inputs, outputs and stack traces?
If a question is difficult to read and is too vague - suggest an edit, comment. Do something about it.