Hmm I’m bit surprised. There are certainly people who play those games regularly. There’s even a Discord community called DOSBox Deathmatch Club centered around DOSBox games in multiplayer: Discord link
What about Doom/Duke 3d/Blood online sessions or even small tournament through the emulation of null modem and IPX\network gaming over networks in eXoDOS project? I still have to try multiplayer by myself but some other people use it regulary with various games.