LABs at the moment are at a strange location in the balance. The majority's opinion is that LABs suck and that it's always better to make a tank.
Spoiler: show
While LABs do suck a bit, they don't suck as much as people seem to think they do. LABs can be very powerful tools on most 5x5 maps and 10x10 maps. They are very cheap and effective expansion engineer raiders and are of the utmost importance to build to gain an advantage.
HOWEVER, LABs do need a buff to their damage. Right now, as it stands, you don't even need to micro a tank to kill a LAB that's heavily microed. This should not be so. A microed LAB should be able to beat an un microed tank, with a microed tank being able to kill a LAB very easily.
The case of "why are LABs not built later in the game" is very simple: they require intense micro. Tanks (with the exception of aurora) can just run in with no worries (ofc if you scout and see no defence, less units, no ACU, etc). The case is much the same as building a T2 range bots instead of a T2 tank. However, LABs should never be used as a direct combat unit against anything, as it is now. They are meant for early raiding, not for combat against a tank force greater than 1 or 2 units.
LABs can be improved though, either their damage (with appropriate nerfs etc) or.... a major MAJOR buff.....
Make them prioritise engies.
Spoiler: show
This will make micro much much easier and as a result buff their raiding capabilities massively.