Because if something can be gained--if you can grow stronger from what you experienced, then there is something of value there. It's the only form of power we have against the Malnosso.

[And one Homura himself places great value in himself, if the seriousness of his reaction is any indication.]

Refusing to face what they forced you to go through is merely another form of bowing down to them. If you can't even face me, then your will is significantly weaker than I expected.