It is technically possible, and some registries publish A or MX records directly at the TLD level. For example
dig A ws. +short will get you 184.108.40.206 and you have an MX record too:
Some are speculating that the rush into new gTLDs is, at least in part, due to people thinking that by having such TLD they would have been able to have emails directly
@tld and a website like
http://TLD/. Imagine if TLD = SEARCH for example.
It would have been basically the rebirth of the concept of keywords that were all the rage long ago, when promoted as concurrent to the domain name space.
This would have created, among other things, a lot of problems on the web due to cookies and the way that there is currently no automated mechanisms for browsers to find administrative cuts (since they do not fell perfectly on the DNS "dot" separator). Same for WPAD DNS auto-discovery and various other mechanisms were applications walk the DNS tree up the root.
Nevertheless it more often than not does not work as expected, mostly due to default configurations. Systems are configured with a search list, so that when you use a name somewhere it is also tried with various suffixes.
This list can be fixed or even retrieved by DHCP.
Also, ICANN restrictions (in the registries contracts) prohibit basically everything at the apex of the TLD, besides
RRSIG records (for the main ones). But that impacts only gTLDs as ccTLDs are not in a contract with ICANN and are free to do what they want on that point(!).
You had a lot of discussions, reports and resolutions around these "dotless" domains, see https://features.icann.org/dotless-domains and especially the technical report: https://www.icann.org/en/system/files/files/sac-053-en.pdf which was followed by an IAB report: https://www.iab.org/2013/07/10/iab-statement-dotless-domains-considered-harmful/
This article is a good summary: http://www.circleid.com/posts/20130711_the_missing_link_in_dotless_domains/
And for many technical details: https://labs.apnic.net/?p=429
with studies on various resolvers on various OSes.