Assuming that the CD-ROM ISO file is a faithful duplicate of the original physical medium, I would look at this from the perspective of Microsoft. They want to minimize technical support costs while supporting a wide product line. This was an era when not every BIOS, hardware configuration supported booting from CD-ROM. So supporting legacy hardware was a safer bet that many users were already familiar with: Booting from floppy. A floppy would provide the essential device drivers to cover the majority of hardware permutations, the goal being to spin up the CD-ROM and mount, read a filesystem.
The OEM version is for a specific range of machines where the devices, BIOS firmware are carefully understood and expected to boot correctly from CD-ROM every time since the installer is likely to be a technician or even a retail employee.
I recall that there were numerous OEM versions of Windows 98. I'm pretty certain I had a retail Windows 98 SE that was a bootable image, but that was a long time ago. I was no fan of Windows 98 and that was the OS that pushed me over the cliff into FreeBSD. I think I remember there being a tool/utility on the disc to make a boot floppy, but I might be remembering the FreeBSD CD-ROM.
Timothy D Legg