Hate to side with Pop an R.C, because there's things I didn't agree with in the playoffs last year and just about every recent playoff run, but during the past 5 years, I'm pretty sure they've tried there hardest to acquire quality bigs with the assets they had available at each point in time during the past 5 years.
For reminder: They traded for Kurt Thomas, they recruited Drew Gooden off waivers, they signed Antonio McDyess knowing they'd be luxury tax payers, they signed Tiago Splitter, they and Tony Parker recruited Boris Diaw and did everything in their power to fit his new contract in with Danny Green and Tim Duncan's this off-season(which some people deemed impossible)--which was smart because Spurs weren't going to be able to attain a better PF in the FA market.
Spurs have done more than you give them credit for the past 5 years,
just because they haven't brought in Dwight Howard or Amare in his prime doesn't mean they haven't done anything to bring in a quality 2nd big. They've tried.