Historic reasons, probably.
RFC 952, published in 1985, with the title of DOD INTERNET HOST TABLE SPECIFICATION mandates that hostnames contains letters (a-z) and digits (0-9), and cannot start with a digit:
- A "name" (Net, Host, Gateway, or Domain name) is a text string up
to 24 characters drawn from the alphabet (A-Z), digits (0-9), minus
sign (-), and period (.). Note that periods are only allowed when
they serve to delimit components of "domain style names". (See
RFC-921, "Domain Name System Implementation Schedule", for
background). No blank or space characters are permitted as part of a
name. No distinction is made between upper and lower case. The first
character must be an alpha character. The last character must not be
a minus sign or period. A host which serves as a GATEWAY should have
"-GATEWAY" or "-GW" as part of its name. Hosts which do not serve as
Internet gateways should not use "-GATEWAY" and "-GW" as part of
their names. A host which is a TAC should have "-TAC" as the last
part of its host name, if it is a DoD host. Single character names
or nicknames are not allowed.
This was changed in RFC 1123, four years later:
The syntax of a legal Internet host name was specified in RFC-952 [DNS:4]. One aspect of host name syntax is hereby changed: the restriction on the first character is relaxed to allow either a letter or a digit. Host software MUST support this more liberal syntax.
So whilst the latter is allowable today, it has not always been so, and I'm guessing it's an convention that is still held by quite a few today, which is why programmers has (wrongly) enforced this.
IMO it could be submitted as a bug towards the installer.
It will work on modern systems, and you should see no issues on modern platforms that conforms to modern DNS standards.