Sometimes it's not the DM who refuses to allow 3pp material into a game session. I can imagine that there have been a number of D&D fans who decided to generate their characters on D&D Beyond, and found themselves unable to use the 3pp material they wanted to use because there was no means to import that material onto D&D Beyond.
I can imagine this is an IP issue. WoTC doesn't want it's D&D Players bringing in material from a 3pp source onto their D&D Beyond website. Especially material that puts theirs to shame.

Hmm...I wonder if you can import material from DMsGuild to D&D Beyond. That would be neat, if it was possible.

WoTC does control the Guild to a certain degree.