Why does the meta title tag have to be a certain number or length of keywords?

You mean why are there keyword limits? I mean, obviously the keywords won't do any good if people type a million different things in that space. The whole idea is to whittle down to the essence of your website or blog or whatever.

yes , better to use like below

Title - The maximum number of characters 80
Description -The maximum number of characters 250
Keywords - characters 300 (better not over 300)

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