You're conflating two things - in one situation, an objective answer exists ('Which database is best for our purposes, for some understood definition of 'best'?

You're conflating two things - in one situation, an objective answer exists ('Which database is best for our purposes, for some understood definition of 'best'?), and the means for identifying that answer are clear (ie, performance testing/profiling), but it's logistically impractical to carry out that process. In the other, there is no objective answer: even the most debated political topics are inherently subjective, because the tradeoffs involved are value judgements (ie, whose satisfaction is worth more? is X/Y/Z immoral?) and there's no process - however slow - that will prove an 'objective' answer to, say, whether abortion/gay marriage is 'moral', or whether it is 'moral' to tax the rich to feed the poor.