Seems like the reason for not treating in general is that human guts are already infested with all sorts of relatively harmless bacteria (and protozoa, etc.). And the drugs to treat toxoplasmosis, which are certain antimalarials and antibiotics, carry some risk. A big risk is that, if you nuke your gut biome with 1-3 antibiotics for everything you get infected with, and they develop resistance which spreads horizontally, it could prevent antibiotics from saving your life when you're infected with something serious.
That being said, if there's a link established between t. gandii infection and suicidality, I'd speculate you'll see lots of pressure from the public health folks for treating all infections.
This is not medical advice and I am not a doctor. I have time to sit in an armchair and argue about toxoplasmosis on a Friday night. Please take that into account.
Once a diagnosis of toxoplasmosis is confirmed, you and your health care provider can discuss whether treatment is necessary. In an otherwise healthy person who is not pregnant, treatment usually is not needed. If symptoms occur, they typically go away within a few weeks to months. For pregnant women or persons who have weakened immune systems, medications are available to treat toxoplasmosis.
https://www.cdc.gov/parasites/toxoplasmosis/gen_info/faqs.ht...
Maybe we should make it more of a priority to treat it rather than be all "Meh. Not symptomatic. Moving on."