Because her daughter can't birth the god in that condition and the game outright tells you it was an accident. (assuming you play any version besides the North American release)
At best you can say it was self inflicted by Alessa sabotoging the boiler but the fact that the game explains the phenomenon is related to stress implies it happened accidentally as a by-product of Alessa panicking.
Dahlia still created these stressful conditions on top of her general torments like bullying.

Kaufmann and Dahlia are more monstrous than the monsters from Alessa's nightmare.
Dahlia only cares about the vessel, not so much her daughter beyond bragging rights that she'll birth their god. She didn't care enough to ease her endless suffering. Alessa cared more for her mother than her mother cared for her. Why else would they show the flashback where Alessa begs her mom to not go through with the ritual near the end?
My criticism is Twin Perfect based their "accident" theory on the opening FMVs which were really just there for artistic purposes than narrative ones. While it may be an accident it certainly wasn't as innocent as they presume. It came from something evil. Why else would she be draped in bandages, blood seeping through constantly? It was a botched pagan ritual most likely.

The reoccuring imagery of puzzles, often paintings you burn isn't just a coincidence. And I don't think it's just symbolic of her just being burned in the same way enemies are, it's all connected to the cult. The cult is at least 80% Pagan in influence. Pagans and children don't really go together well if you know history.
the game explains the phenomenon is related to stress implies it happened accidentally as a by-product of Alessa panicking.
"hmm today I'm going to torment a psychic child with demonic rituals in a spiritual town, what can go wrong?" "Let's also let her know she will carry the burden of being the mother of God too." -the cult before SH1
I don't even think I need to explain that SILENT HILL happened because of the cult. Pagans are that bad. They were involved in the White Claudia incident, murdered officers who discovered their conspiracy, drugged innocents, tortured a psychic child which resulted in the nightmare mixing with whatever spiritual power the town already had, or in SH1-only terms reality being invaded by her nightmare made manifest. They still pursued Cheryl long after SH1's events and killed Harry to lure her to her "destiny". If Alessa was raised by someone who loved her she would have never been like this and her nightmare and stress would have been nothing. And that's also a contrast between her and Heather (Cheryl) when she reaches the same age Alessa was.