Older versions of Android used to allow mounting as a mass storage device, but there are several problems with this:
- You can't allow apps on the phone access to the filesystem at the same time as it is mounted as a mass storage device (because you may corrupt the FS).
- It doesn't work with full-disk encryption used by newer Android versions, because your computer doesn't have the key (and the key is probably stored somewhere in the TPM of your phone, encrypted by your password).
- It requires the computer to support the FS used by the phone, which means necessarily using something that Windows/macOS supports (which mostly limits it to FAT/exFAT/NTFS)
So starting from Android... 5 or so? you can't mount as mass storage anymore. This probably consists of 99.5%+ of currently functioning Android phones.
Mass Storage devices are attached as block devices. MTP devices are attached as file devices. As long as the OS understands the filesystem on a block device, it can mount it. For file devices, it needs to speak the protocol (in this case MTP). MTP is similar to how Apple handles attaching of its devices, just a different protocol.
Mass devices can be easily corrupted and have to be unmounted from the current device. MTP can stay mounted and accessible by the mounting OS and source device simultaneously and are much more difficult to corrupt.
It also allows the host device more security and control over what the guest can do.