Because, we donot have access to the hosting space. So we cannot setup robots.txt, the atom.xml files are sometimes not acceptable by Google Webmaster tools and we cannot create one and submit our own. Since they are subdomains, some search engines/directories may not accept their inclusions..
I have no idea what you are trying to say. Are you saying you cannot rank with blogger? Of so, you have bigger issues that worrying about robots files!
I dont see what is the problem with getting your own domain and doing everything just as you like, unless you have lots of traffic and good positions to that blogger site.
What I meant is, in blogger.com we donot get access to the hosting space. So, we cannot upload the google verification file, you cannot create and put a robots.txt file and since we get subdomains like site-name.blogspot.com, some directories may not accept them. I created a website with a domain (buying a hosting package) & also a subdomain.blogspot.com site at the same time. My domain has a rank of 800,000 now and the blogspot is still at 11,000,000. Thats why I was thinking since its a blogger account, it is not properly getting SEO-d.