> Because some users still have 32-bit computers. How is this not obvious?

It is completely not obvious that there are Windows 10 users with 32-bit computers.

Here's an analysis from Debian trying to figure out whether to default to the 64-bit image: https://lists.debian.org/debian-devel/2017/06/msg00059.html

    No one installs i386 new -- machines that are non-amd64-capable are:
    * mainstream machines from 2004 and earlier
    * a brief wave of mobile devices, now thankfully gone
    * deeply embedded, usually incapable of running [the standard Debian installer]

From the followup comments, there were apparently a few netbooks using the mobile chipsets as late as 2012, but I doubt a netbook from 2012 using a mobile chipset is going to be well-supported by Windows 10, regardless of CPU architecture.