I wonder why languages lost that form, because it seems really useful to have a single verb for those.

I am not sure, but I think it’s due to the changes in the passive. Latin had proper passive forms for plenty verbs, and a lot of those verbs handling states were either deponent (passive-looking with active meaning; like irascor) or relied on the passive for the state (like terreo “I terrify” → terreor “I’m terrified”). Somewhere down the road the Romance languages ditched it for the sake of the analytical passive, sum + participle.

I’m saying this because, while irascor died, the participle survived in e.g. Portuguese (Lat. iratum → Por. irado, “angered”). And it got even re-attached to a new verb (irar “to cause anger”).