We do have a standard. Most EV manufacturers use the 18650 battery cells, which are cylinders 18 mm in diameter and 65 mm long. You can buy a wide range of 18650 batteries from different sources.
The problem is that those have to be assembled into packs for vehicular use, and it's not really practical to develop an efficient standard form factor for packs given the enormous variance in vehicle designs. When a pack suffers collision damage then in theory it could be possible to reuse any undamaged individual 18650 batteries in a replacement pack, but in practice this is just too risky and labor intensive. So, the entire pack has to be replaced.
The problem is that those have to be assembled into packs for vehicular use, and it's not really practical to develop an efficient standard form factor for packs given the enormous variance in vehicle designs. When a pack suffers collision damage then in theory it could be possible to reuse any undamaged individual 18650 batteries in a replacement pack, but in practice this is just too risky and labor intensive. So, the entire pack has to be replaced.