I collect my waste fork oil in a container. Tonight I ran out of wet lube. The waste 7.5wt fork oil seems very similar. Can anyone give me a good reason why I shouldn't use it on my chain?

Many fork oils have a solvent content which aids application, but also makes them seem runny like light fork oil. The solvent evaporates after application, leaving a coating of none-drip lube. The fork oil will remain thin and although it will lube the chain, it will be quite messy and may fling off onto frame and clothing. Any lube is better than none 😉