The trouble is that we only get one vote at each general election, and those normally only happen once every few years. That isn't even close to enough to express any sort of nuanced view on important issues like civil liberties. It's not even enough to express a clear view on headline issues like economic policy or the NHS. And yet politicians produce these gazillion-page manifesto documents and the winners then claim a public mandate to implement the policy mentioned vaguely in the footnote on page 87.
Our political system is fundamentally broken, and it's not (only) because of FPTP. In many cases, there may not be any potential representative you can vote for who is even close to your own position on even most issues.
Saying that people "don't care" about an issue because they don't vote for a party that opposes the problem on that one issue makes no sense. For example, in the case you mentioned, the Lib Dems have essentially made themselves a single-issue party this election. By your own reasoning, should someone who supports their stance on civil liberties but disagrees with their headline position on Brexit vote for them?