Maintaining a stable chain reaction is surprisingly difficult. The neutrons that come from out of the fissioning heavy atoms are going way too fast. The neutrons aren't blasting into the other atoms so much as merging with them and causing them to become unstable. If the neutrons are going too fast, they just bounce off and nothing happens.

Yes, putting fuel pellets close to each other increases the odds that the neutrons will bounce around, slow down, and then cause another atom to split. However, unless you just put a lot together, that won't happen - at least, not at a rate that maintains the chain reaction.

Nuclear bombs get around this by using conventional high explosives to compress the nuclear fuel while also setting off another nuclear fuel with a much lower half-life that spits out a huge number of neutrons. Enough neutrons are released and the nuclear fuel has been compressed enough that even with a pretty low chance of the neutrons causing more atoms to split, enough of them split anyway and boom.

Nuclear power plants use a moderator like water, which slows the neutrons down without (hopefully) absorbing too many of them. The pellets are separated by the moderator so that as a few atoms naturally fission the neutrons are slowed down by the moderator. When they get to the other fuel pellets, the neutrons are going slow enough to have a high chance of interacting and causing more fission - releasing more neutrons, etc. The process is controlled by controlling how much of the fuel is exposed to the moderator and other fuel pellets, and by controlling "shields" made out of a material (like boron) that absorbs the neutrons without doing anything, blocking the chain reaction.