Because goth culture is 100% superficial, anyone can be one. You just have to adopt any one of the "officially certified gothic appearances" and pretend to like the same music other goths do if you're out at a club or something (it helps if you actually like it, but if you can fake it, that will do). In real terms, anyone can be a "goth" just by dressing up on the weekend, it's actually not that different to cosplay and can be a lot of fun. Girls who are a bit self-conscious about their figure really like it in particular because they get to justify wearing corsets in public. This has its benefits.
Of course, people deep into a gothic subculture tend to have a really snobby attitude and therefore find the idea of "normals" infiltrating their group a bit abhorrent. Part of how goths as a social group operate is by insisting that other people "don't understand what Goth really is", which then allows them to include or exclude whoever they like based on arbitrary rules concerning music, fashion and appearance - items in goth culture which have social value and exist primarily as a mechanism for inclusion/exclusion. This is a social sleight-of-hand because Goth isn't really "about" anything once you dig beneath these social signifiers and that's one of the things that defines it. For example "real goth music" doesn't really exist on any sort of musical level. There is no functional, measurable musical difference between The Cure's "A Forest" period and a lot of other equally maudlin pop music that was being produced at around the same time, but the difference does exist aesthetically (hair, fashion), and also ethnomusicologically, because it is the "tribe of certified goth opinion" which decides what is "real goth music", as opposed to any objective standard which can be measured in musical terms. This can vary from region to region and also depending on who they think is trying to infiltrate the group. In the 80s pop music was close to what was considered goth at the time, so it was the pop fans who were excluded. Nowadays it's metalheads and emos that they don't like, because some metal and emocore is veering towards themes that were once exclusively the domain of goths - tell a group of goths that you like Evanescence or My Chemical Romance and watch their heads spin around like Linda Blair. You would actually get a better reaction from them if you say you like Madonna.
Yes there are exceptions to everything stated above, but I have observed everything that I have written about here first-hand.
