why need theme,if page builder is there in wordpress [closed]

Because is it a true s**t experience having to design every single page of your site. This can be ok if your site contains up to 10 pages which rarely change, but it is very bad ROI if you need to spend that much effort to publish a daily blog post.

In general the most important aspect of wordpress development is automation of admin tasks, and while there is no obvious and immediate contradiction between that and page builder, the reality is that most of the page builders existing right now are not coded well enough to support automation, and are lacking standardization which is a must for automation.

Anyway 4.8 is supposed to have a sort of page builder included in core…. will be interesting to see how well it work for all the various ways people use wordpress.