Account creation was launched back on 9 November 2012.
As a feature, and in line with releasing early and often, it was released with the bare minimum to work.
What the sign in form lacked at that stage was an option to let you stay logged in.
The option to stay logged in is a very common feature and rightly so. Without such an option, you will remain logged in only as long as your browser session is active.
In general, you're not helping your users if you require them to sign in every time they re-visit your site.
This is not quite the case with Simply Testable as if you're not signed in you get, at least currently, access to more or less everything you do if you are signed in.
This won't always be the case.
In the future there will be paid plans offering a wider range of benefits as well as free plans offering you greater benefits than someone who is not logged in at all.
I also noticed, when recently examining what data to regularly back up, that very few full-site tests were run by users who are signed in. Just 2% at present.
That most users of Simply Testable don't create accounts and don't have the option to sign in may well explain the low number of full-site tests run be signed-in users. That you did not, until now, have the option to stay signed in may also have been relevant.
The good news is that you can now choose to remain signed in. As we roll out more benefits for signed-in users over the coming months you will, as a signed-in user, get more value out of using the Simply Testable web testing services.