Contemporary governments deal with taxation, healthcare, security, defense, education, law, labor rights, minority rights, infrastructure, prison systems, regulations of industries, and so on and so on and so on. It’s very unlikely to find one person capable of having in-depth knowledge of all of these areas to properly defend their party’s leanings on all of them in parliamentary debates, and even if you did, those parties are still going to need experts who draw the master lines of their policy proposals, and those experts need to be paid.