Yes, and I think the way that Web 2.0 sites (as defined by the way you are using the term, Jubaer), including those hosted on Wordpress.com and Tumblr, is a great example of why search engines treat subdomains as distinct domains.
Practically anyone can set up a site on Wordpress.com like abc.wordpress.com with minimal effort. That is, after all, the primary service Wordpress.com offers, after all. The sites that exist as subdomains of Wordpress.com have very little in common with each other, other than being hosted on Wordpress.com.
Wordpress.com has a DA of 96. There are some truly fantastic sites hosted as subdomains of Wordpress.com, and there are also a lot of really crappy ones, too. If search engines treated the value of a link from crappysite.wordpress.com as authoritatively as the value of a link from fantasticsite.wordpress.com simply because both are hosted on wordpress.com (which does, as you say, have a DA of 96), would that be best for users? Nope, it would not. Someone could have made crappysite.wordpress.com for the sole purpose of "earning" a link to their own (self-hosted) site, or created it with good intentions, but never put in the work needed to become a valuable site for its intended audience like fantasticsite.wordpress.com did.
And, let's not forget about the importance of relevance. The topics of sites on Wordpress.com are all over the place. Therefore, it makes sense to treat each one separately, right?