Why is the Peltier / Seebeck Effect's efficiency so low in practical devices?
Here is why.
To be efficient in this context requires two contradictory properties: 1) that the thermoelectric material have low thermal conductivity (so the temperature difference across the junction is not thermally short-circuited) while at the same time having high electrical conductivity (so the cooling effect is not swamped out by I^2R losses in the junction).
To the extent that these properties are both affected by things like electron mobility, you can't make one low and the other high in the same chunk of material.
You can finesse the problem somewhat with exotic materials from obscure regions of the periodic table, which is why solid-state refrigerator junctions are made from things like balonium-doped canthavium/unobtainium alloys. This is a materials science joke; you may laugh now.