TL;DR: this is e-mail legacy.
Neither GnuPG, nor Thunderbird, nor probably any other implementations of OpenPGP will bother about the line length. But the whole e-mail infrastructure does.
Historically, the limit to 78 characters (RFC 2822) came up because of the usual limitations of characters that fit on a screen per line. This is long gone since; while something around this number of characters, or even some fever will still result in a better readability than long lines from the left to the right of your 28" 4K screen, we've got slightly more intelligent and user-friendly software that breaks lines automatically at reasonable limits.
But e-mail is old, and so are (still running) e-mail servers and clients. There are still some in the wild that do not support long lines, because they've never been needed (as they should not occur), and they (might) fail at handling such mail, or they add arbitrary newlines themselves (which will break signatures).
Handling of Long Lines in OpenPGP
For this reason, the "e-mail safe" ASCII-armoring (radix-conversion) of OpenPGP wraps at (maximum) safe 76 characters per line. On the other hand, clearsigned messages will not be rewrapped in the clear text section, thus Thunderbird/Enigmail remember you to stick to that character limit.
OpenPGP/MIME-encoded mail (and MIME-encoded mail in general) does not know this problem, as the quoted-printable encoding takes care of adding required linebreaks, which are removed before the message hash is calculated for verifying the signature.