Because he’s worked very, very hard for decades to get his career to where it is, and I’m sure he’d like to keep it that way. Do you know of any openly gay (or bisexual) actors who are frequently cast as the lead in movies by top directors? Do you know of any openly gay actors who have been nominated for seven Oscars, mostly Best Actor, and received the award once? You don’t, because there aren’t any.
He’d probably never again be cast as a character who has any type of female love interest. Titanic would develop this weird sense of irony in people’s minds - “well, you should have let go, Rose! He was gay!” While Leo may be middle-aged now, there are two whole generations of women who remember swooning over him in their preteens, teens, and twenties.
It could be a concern on Lukas’s side, too. He’s not famous; his best-known role is probably still as the little Amish boy in the 1985 Harrison Ford movie Witness. (He and Leo are both former child stars.) But he works pretty steadily, and he might want to keep the range of roles he’s considered for open. Not to mention, people would probably mutter, “He just got that part because he’s sleeping with Leonardo DiCaprio.”
It’s odd and almost without precedent for a famous and renowned Hollywood actor to be as private about his personal life as Leo is, and for it to have such a lack of public milestones. No engagements, no wives, no children, no long-term high-profile relationship since he dated Gisele way back in the day. You ever notice how these models never really say anything about their relationships with Leo? Not even anything like, “He’s a great guy, but we were just too different”? There are probably NDAs in place.
Also, if people knew Leo had been in a long-term relationship with Lukas Haas for years and years, he’d look like a fraud, and these models would look like underhanded and shameless in their pursuit of their elevated public profiles.
I do think it’s a very long relationship, too. If you do an image search for Leo and Lukas together, you can see that they go just about everywhere together and have for years.