It has an opportunity cost. It's not burdensome to be challenged, per se, it's burdensome to repeat that, "Yes, I think trans people are humans with complex interior lives who mostly want to be left alone. No, they are not coming into bathrooms en masse to molest you. Yes, they deserve to just get on with their lives. No, we shouldn't mock them for who they are" etc. etc.

It takes someone 3-5 seconds to "just asking questions" and it takes me much more than that to respond. There's an obvious imbalance there that leads to:

a) It's a lot more exhausting to respond than to ask

b) It's vulnerable to malicious askers abusing my good faith answering time

So while, with close friends, I'm happy to answer questions. Or with well meaning allies who genuinely want to learn and just don't understand. But like, random folks? On an internet forum? Nah. They can ask and I can say, "Sorry, let's just not engage."