When this decision was made for MS-DOS (and it may not have even been made then, but simply brought in from 86-DOS which may have cargo-culted it from CP/M-80) it is very unlikely that "what ARPAnet requires" was a consideration. Networking was not included with microcomputer operating systems at that time and network compatibility between systems from different vendors was an exotic requirement on top of that. Most microcomputers in the late 70's and early 80's either only talked to their own kind or were (more commonly) not networked at all.

People today forget or don't realize how provincial and siloed computing used to be. Even as late as the 90's it was very common for institutions to have multiple LAN's that were isolated from each other simply due to platform incompatibility.