Ukrainian grain exports used to be tens of millions of tonnes per year. That's something like a million semitrucks or a million railway cars. You can't build that kind of capacity in a few months, especially not in the middle of an all-out war.

As long as Russia can operate submarines in the Black Sea, it can blockade Ukrainian ports. No foreign cargo ship will approach the ports, and no foreign insurer will insure such ships.

Ukraine also depended on Russian energy. If Europe wants to phase out Russian energy imports, it can't supply enough fuel to Ukraine. Meanwhile, Ukraine has better uses for the fuel it can get than exporting low-value bulk goods. Like keeping its military operational, which also needs the trucks that could export grain.

Ukraine uses the Soviet 1520 mm rail gauge, while its western neighbors use the 1435 mm gauge. That creates a huge bottleneck for rail transport. Not to mention that Russia has been targeting Ukrainian railway infrastructure.

There are not that many major roads from Ukraine to the west, and some of them are unusable. The border between Moldova and Ukraine is largely controlled by Russian-backed separatists, while Russia has been bombing the bridges on the only road south of Moldova repeatedly.