[@AlternateMan] Two problems with 'Let's do this instead of bombing them to hell.'.

{1} It doesn't hold up forever.  Even the blobs would be expanding and contracting to break out.

{2} Even if you did this instead of bombing the planet, you're...what?  Burying it in concrete?  That still ruins it, AND it doesn't actually KILL the X.

The 'crete was probably used to help with evacuations wherever possible, not as a [i]solution[/i] to the problem.