Standard enthalpy is the heat given off or required for a chemical reaction carried out on one mole of the substance under standard conditions - that is 25 degrees Celsius and one atmosphere pressure. No more no less. We said energy was the sum of heat given off and work done. How then can we measure energy by determining heat effects and ignoring the work. The reason is that we define work as a force acting over a distance.
Gas expanding in a cylinder, against a constant resisting pressure, is work.
A gas expanding into the atmosphere with no restraint is not work. In the absence of work, all the energy appears as heat. We can measure heat rather well, so standard enthalpies are collections of heat measurements done under the conditions of no work. But they are measurements of energy none the less. So thermochemistry is limiting.