Think of it like a bunch of dominoes standing on end - and "half-life" is how long it typically takes half of all independent dominos to tip over because of a tiny breeze, etc.

If these dominoes are set up very far apart, then one domino randomly tipping over only releases a tiny amount of kinetic energy and isn't likely to knock any others over. This is where the natural independent "radioactive decay" idea comes into play, and is how "half-life" is defined.

On the other hand, if there are very many dominoes standing next to each other, then one domino tipping over might push two others, which might push into four others, with might push into eight others, etc. All of these dominoes (except the very first one) didn't "decay" naturally, they were all pushed! So, the amount of fallen dominoes is more a question of "How closely packed are the dominoes?" and "How long on average does a chain-reaction of domino falls last?" rather than a question of isolated "half-life".

Nuclear reactors burn through fuel quickly compared to the fuel half-life because they are designed to sustain those long chain-reactions that release more energy faster.