A short notice assumes someone else keeps spare capacity in their life.

Short notice quietly assumes the other person has slack. Asking for something tomorrow only works if there is unused capacity in their life, and people vary enormously in how much they have. Those with plenty tend not to notice that they are asking; those without it often say yes anyway and pay for it later in ways nobody sees.

When you have to ask late, say so out loud and make refusal easy. A request that names its own awkwardness costs far less than one that assumes availability. And on the receiving end, remember that not having spare capacity is a fact about your circumstances rather than a failure of generosity.