If those were truly mergers, I wouldn't have a problem.
Rather these are acquisitions, for pretty good money. Money which has to be borrowed, saddling debt on (usually) the acquired company.
The end result is that instead of re-investing profits in R&D and equipment, those profits are used to service debt.
"Cost reduction" efforts become the over-riding company focus. People leave or are let go.
The end result is that the acquired company is decimated, and the resulting conglomerate is less than the sum of its previous parts.
Having said this, ADI actually has done a good job with Linear Tech, it has allowed it to maintain some modicum of corporate identity. LTSpice has not been replaced by ADSpice.
Not so with TI, which has completely digested and vanished National. Among others, like Burr-Brown and Unitrode.