Because batteries are HEAVY. A modular design means the batteries can't be structural, so you need more structure to support the car around the battery, and a solid structure holding the replaceable battery itself. Boxes in boxes in boxes in boxes. This makes the car heavier. Weight means less range. To increase the range you need more batteries. Which adds more weight. Which means less range.
Do you see what I mean? It's just physics. We can't fix this. Only some amazing battery breakthrough would fix this, but so far we have only sensationalist articles about something working in a lab supposedly and nothing out there in production.