Strangely I did the same thing last night and it did resume, although you only see the resumption when it starts. There's no prior indication that it will resume.
Having attempted (unsuccessfully) to write a resumable HTTP/HTTPS downloader, which is what I suspect nsurlsessiond is using behind the scenes - it's really hard to get it right. Meanwhile I expect to be able to start a BitTorrent download, throw the laptop down the stairs, take out the hard drive and be able to successfully resume it on a different computer because that's a protocol that was actually designed for it.
https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requ...
And if they need to get more clever than that, why not have a BitTorrent-like map of chunk hashes. So you download the update, check the hash of the whole thing and if it fails the hash check, download the chunk hashes which will be say a 256 bit hash for every individual 32 MB chunk of the file. And for any chunk that fails the hash check, use a HTTP range request to download it again.