VTE and, with it, almost every Linux distro, will get Sixel support soon. I volunteered to add Tektronix graphics to it too, but this is neither a dire need, nor something I have done before, so it'll take some time.
Because things that existed 40 years ago are useful, already have software written for it, are compatible in sometimes unforeseen ways (a DEC dot-matrix graph can be printed as is on a Sixel-compatible terminal!) and have been battle tested for ages.
There is a reason the Unix way of bytestream-based shell and pipes is still useful and present these days to the point that That Other OS is now embedding Linux in it.
Also, these ancient terminals often had some interesting typography options that are encoded in the ANSI standard that most modern terminals don't bother (line attributes that generate wider and taller cells are one such example).
These formats may be more desirable than more modern and complete ones such as PostScript for other reasons. I wouldn't advise implementing a terminal capable of rendering PostScript graphics because it's one more way to infiltrate malware in your computer by rendering untrusted inputs (There are a lot of RCE opportunities in exploiting vulnerable decoders).
Why are we not focusing our energy on making something that is actually up to date?