It's definitely not ideal that you can't host content on the root domain, especially for migrating sites. The reasons we don't currently support it are partially due to internet infrastructure and partially due to reliability concerns.
We rely on CNAME chains to route domains to our servers. Unfortunately the DNS specification does not allow CNAME for root domains. There are ANAMEs which do allow CNAMEs for root domains, but that's supported by very few DNS providers.
The alternative to CNAMEs is just using IPs which DO work for root domains. The issue with IPs is that if they are DDoSed, then customers have to update their DNS to switch to a new IP to restore their site. Since it's not feasible to give every customer their own unique (v4) IPs (or set of IPs), a single DDoSed IP could take down many sites. In fact, many years ago we used to give out IPs, and we suffered this exact situation.
We do offer a service to automatically redirect your root domain paths to www domains so you don't have to set those up manually with another hosting provider.
We're also investigating new tricks that may allow hosting on the root domain, but we don't have a timeline for that at this time.