So in my experience, there's a few factors to how this comes about. First, to actually be supportive, people must toss out several assumptions that come with cisheteronormativity; most importantly, the parts of it which Others those who don't conform to cisgender heterosexual cultural ideals. If you are no longer seeing trans folks as Others, to some degree it becomes a thing you yourself could be, as it is no longer strictly in the category of things which are for other people. This alone gets over one of the biggest hurdles of identifying oneself as LGBT by contributing towards breaking down the internalized transphobia and homophobia built up by society.
Secondly, folks tend to hang out with people similar to themselves; those who gravitate towards being in groups of queer folks are much more likely to be queer, if even they themselves haven't realized it yet. Among the social circles I've observed/been in, queer groups have also tended to be relatively neurodivergent, and accepting of such, and as a result, people who enter and stay in the groups are queer and/or neurodivergent. This results in groups consisting of a bunch of queer folks, many of whom are neurodivergent, and a few who aren't particularly queer but love that they found a group of people who enjoy listening to their fascinating special interest rants (some may even be in the otherwise queer polycules that often form from these). A good number of these often also end up as queer at a later time, but generally not all.
People can also be supportive without being in these sort of circles. The problem is, it's not all that common even among people who think they are supportive, just because their idea of 'supportive' is divorced from the reality of actual people they believe themselves supportive of. Most people falling into this pattern of 'support' just want to be patted on the back and applauded for quietly saying 'genocide is wrong' to themselves in a locked room, instead of taking actual actions like looking up #transcrowdfund on twitter and saving a life with some of their leisure budget, or even just educating themselves and putting in enough effort to actively make trans people comfortable in spaces they are in while making transphobes too uncomfortable to stick around. Thoughts and Prayers are neither support nor supportive, in spite of the weird cultural belief that they are. Tbh, people who are actually providing material support in some way but are neither queer, nor in queer circles, nor materially obligated to do so, are a minority about as small as trans people. This is a pattern which extends beyond LGBT stuff and into the realm of other minorities as well.