If you made memory access lines twice as wide, they’d take up more space. More space means (a) chips run slower, because it takes time for the electricity to get there (b) they’d be bigger and more expensive.

The main problem with 32-bit, as others have noticed, is that that’s not really so much RAM. CPUs do addition and subtraction the way we were taught at school - ‘carry the one’, they’ve an overflow bit that’s set when your sum doesn’t fit in the columns. On 8-bit CPUs, we were always checking back when adding up large numbers. On 64-bit CPUs, we can deal with truly massive numbers anyway, it’s not such a hassle. And they’re so fast at doing sums anyway and usually waiting for memory, it’s barely a hassle.

Moving to 128-bit would give us a truly minuscule, probably unmeasurable, benefit in exchange for significant downsides. We could make them, but it would be pointless.