the same tools that would trivialize creating such a virus would also trivialize a cure.
Additionally, cures by their nature will always lag behind diseases, people won't invest resources curing a disease that doesn't exist. A non-trivial amount of damage would occur before society reacted to the threat.
Also, it's not illogical for there to be a demand for (research into) protections against threats which have not materialized yet, but seem to have a chance of doing so. As the prospect of portable bioweapons becomes more likely, there will be increased incentive to come up with protections from those weapons, even if they have not yet reached viability.