You and I can speculate the reasons but I don't think we will know why virtual BIOS in VMware does not support USB boot; unless some current/former VMware employee(s) who has inside knowledge will come to share the story (highly unlikely).
Sometimes, features of a software is not just dictated by technology but also by business/contract reasons.
For one thing, it would look that VMware would have more control/flexibility in adding features to Virtual UEFI versus virtual BIOS. If you look at msinfo32 in a Windows VM, you will see "Phoenix Technologies" for a "BIOS Version/Date" with virtual BIOS while one with virtual UEFI shows "VMware Inc"; which suggests that virtual BIOS was licensed from Phoenix (possibly with modification restrictions or without source code) while virtual EFI was made by VMware. So to add USB boot support to virtual BIOS might have meant Phoenix Technologies modifying the virtual BIOS and/or VMware possibly having to pay additional fees for a Phoenix BIOS (or another BIOS vendor) with such a feature. This is also an example that you and I can speculate the many possible reasons but will never ever know...
Anyway, Intel has announced in a presentation in 2017 that BIOS will no longer be supported by 2020 but there is no clear roadmap of how this will happen. Even before this announcement, some add-in graphics cards don't even work with physical machines with BIOS (ie. the machine won't boot with the card installed) and works exclusively only with UEFI. It would be best any new VM(s) you create use virtual EFI or seriously consider converting existing VMs with BIOS to EFI.