Liquid sorces lack other nutrients, fiber, and can give you gas to no end. Furthermore, depending on the person can also induce allergies (Note that I said it is person specific). Finally, Liquid protein sources are generally quickly digested, whereas meat, and eggs take much longer to digest, and so are in effect “slow release”. So timing is important, because post-workout you want to maximize uptake, and liquid sources can be immediately taken up. But throughout the day, you’re looking for a slow, more constant release of amino acids, which would be more available through food sources.