So, logically, spacecraft in the frictionless environment of outer space
should immediately accelerate to the speed of light.


Some haphazard math here.. if space is indeed entirely frictionless,
which I highly doubt, then to accelerate a body of weight 1kg (2.2
lbs) to the speed of light (using a constant force of 1N (or 1 m/sec2
acceleration) would require a distance of 4.5*(10^16) meters or about
45000000000000 km which is about 300 billion miles. The work done/
energy needed would be about 450 trillion joules.
The time needed to achieve this feat would be about 9.5 yrs.. so no
its not instantaneous