Sometimes I am eager to see the activity of other users, who I am interested in . Is there any good way, except to search the id each time, to achieve this? Why can't stackoverflow have the follow feature?
There is an RSS feed on each user page (user feed link at the bottom of the page, to the right) - you can use that to see what they are doing on the site, to a limited extent.
Why can't stackoverflow have the follow feature?
Because Stack Overflow is a Question and answer site. It is not about the users - it is about the content.
In other words - Stack Overflow is not a social networking site and will never become one.