Mostly because most of the times the sexuality of a character in a videogame is simply irrelevant. I do not assume that every character that is not clearly stated as LGTB is heterosexual, i simply don't care or think about their sexuality at all, their purpose as characters is to tell a story and their sexuality not always plays a role (in fact, it doesn't most of the time).
That's why it can feel forced sometimes, because they tell bad stories with those characters just to point out that the character is gay or even worse, they simply announce that a character whose sexuality has never mattered is gay, with no background or story to support it, just to have that diverse quota that it's actually good for business. And yes, the same can happen with heterosexual characters when their sexuality has never had any kind of presence in the game and it doesn't really matter for the purpose of said character, i always thought that about Thrall's marriage for example.
And just to be clear, Pelagos is not this case at all, i believe that's a good story told in a good way.