Because no Atium mistings exist anymore, and atium alloy mistings have never been known to exist either. Nothing to steal from for the spike except mistborn, and that's not an available solution.
Anyone can burn lerasium so that's pointless, though I guess it's unknown whether the allomantic effect of lerasium will occur in those cases. In any case, burning the lerasium automatically makes 16 of the spikes redundant so it is still pointless to consider it.
Atium and lerasium ferrings probably don't exist so good luck tracking down two full feruchemists.