Lifetime is the issue. ATP and its cousins, and similar energy-transmitters like creatine phosphate, are hot potatoes. The phosphate group rapidly transfers its energy to other molecules in the cell, and exporting it to the bloodstream just wouldn't work, it'd break down in transit.
> "In human beings, for example, the amount of ATP recycled daily is about the same as body weight, even though the average human being only has about 250 grams of ATP. Another way to look at it is that a single molecule of ATP gets recycled 500-700 times every day. At any moment in time, the amount of ATP plus ADP is fairly constant. This is important since ATP is not a molecule that can be stored for later use. "
Most of energy biochemistry can be viewed as a system for trapping energetic short-lived species (such as high-energy electrons produced by cytochrome oxidases or the photosynthetic reaction centers) into slightly longer-lived systems (such as protein gradients across a cell membrane) which in turn are converted to somewhat more persistent species (ATP and NAD(P)H) which circulate within cells), and then into the most stable energy stores, like glucose, lactate, etc. which circulate through the bloodstream and transfer energy between cells (with glucose eventually being stored as glycogen in the liver, and eventually, as fatty tissues, or in plants, starchy roots). The times go as nanoseconds -> milliseconds -> seconds -> minutes/hours/days.
> "In human beings, for example, the amount of ATP recycled daily is about the same as body weight, even though the average human being only has about 250 grams of ATP. Another way to look at it is that a single molecule of ATP gets recycled 500-700 times every day. At any moment in time, the amount of ATP plus ADP is fairly constant. This is important since ATP is not a molecule that can be stored for later use. "
https://www.thoughtco.com/atp-important-molecule-in-metaboli...
Most of energy biochemistry can be viewed as a system for trapping energetic short-lived species (such as high-energy electrons produced by cytochrome oxidases or the photosynthetic reaction centers) into slightly longer-lived systems (such as protein gradients across a cell membrane) which in turn are converted to somewhat more persistent species (ATP and NAD(P)H) which circulate within cells), and then into the most stable energy stores, like glucose, lactate, etc. which circulate through the bloodstream and transfer energy between cells (with glucose eventually being stored as glycogen in the liver, and eventually, as fatty tissues, or in plants, starchy roots). The times go as nanoseconds -> milliseconds -> seconds -> minutes/hours/days.