Because all hi-fi equipment and electronic devices contain parts that change over time, wear and need periodic cleaning, lubrication and adjustment.
When you combine the sometimes hundreds of electronic parts with mechanical components like the levers, switches, belts, gears, pulleys and connectors found in many types of hi-fi gear, you have a combination of parts and systems that wear and need periodic attention and maintenance, over time.
Mechanical linkages, pulleys, gears, idlers, pinch rollers and motors need cleaning and lubrication and this is fairly easily understood. But electronic parts also change over time, most notably electrolytic capacitors, some semiconductors and resistors. One thing is for certain though: your hi-fi equipment will need money spent on maintenance if it is to keep running reliably.
Proper periodic maintenance prevents premature failure, improves performance and reliability and therefore enjoyment of your equipment. It’s a quirk of human nature that we sometimes purchase things and expect that they will run without attention. This isn’t even true even for a kettle, let alone complex equipment like turntables, amplifiers and cassette decks.