The activation energy for the hydrolysis of the gamma phosphate of ATP is fairly high which prevents the hydrolysis from occurring while the ATP is just floating around in the cell. So, while ATP may be thermodynamically unstable (i.e. it is a high energy compound), it is kinetically stable (the hydrolysis occurs at a very slow rate). Hexokinase, however, can get around the kinetic stability of ATP by lowering the activation energy of the hydrolysis. Hexokinase can do this because it is an enzyme, and all enzymes act as catalysts by lowering activation energies.
The sittuation can be likened to that of diamond. Diamond is a thermodynamically unstable compound because graphite is the lowest energy form of carbon under standard temperatures and pressures. However, one can say that "diamonds are forever" because diamonds are kinetically stable; the transition from diamond to graphite has such a large activation energy barrier that it occurs at a negligible rate.