They might not believe that and just be trying to gaslight/troll.
They might have heard it's trendy and gets you cool points and somehow don't see how hard it could be.
They could just think trans people don't exist, therefore all must be fake.
They could think you are a troll trying to make trans people look bad.
If you are an NB, it could be someone who doesn't believe in NBies or certain subsets.
If you aren't trying to transition medically, they could be transmeds.
If you suffer from things like depression, they could think you are trying to look for an excuse to blame that on.
Could be someone who is in denial themselves and part of their denial is believing certain things are normal for their agab and trans people saying it's part of their trans experience challenges that. Similar to neurodivergent parents who deny their children's neurodivergent because the parent think of themselfs as neurotypical.
All kinds of excuses people could have for their beliefs. Pretty much all of them boil down to some level of transphobia.