Difference between .zip and .zip64

If people cared to make that distinction then they can, and there is nothing really stopping them. It would certainly help with ancient software that doesn't understand Zip64 format files, but that software should be quite rare these days. It boils down to whether enough people actually care enough about it, which likely isn't that many people when simply using a zip tool updated since 2001 should be enough.

From The Zip version history

• 2.0: (1993) File entries can be compressed with DEFLATE and use traditional PKWARE encryption (ZipCrypto).
• 2.1: (1996) Deflate64 compression
• 4.5: (2001) Documented 64-bit zip format.
• 4.6: (2001) BZIP2 compression (not published online until the publication of APPNOTE 5.2)
• 5.0: (2002) SES: DES, Triple DES, RC2, RC4 supported for encryption (not published online until the publication of APPNOTE 5.2)

"Old" zip and zip64 are still both part of the Zip Specification.

File extensions are far more convention and agreements between people than they are hard identifiers of documents for computers. Many formats get updated over the years but still retain the same file extension.

A person needs to know "a zip tool will open this". The computer will look at the actual data within the file to work out exactly what kind of file it is dealing with and how to open it.