Because the colours are standardised and embed important information. These are tools, not aesthetic objects. A lot of the palette is preserved from paper chart

Because the colours are standardised and embed important information. These are tools, not aesthetic objects. A lot of the palette is preserved from paper charts where they need to be readable under red light when underway at night. For a masterpiece of technical documentation dig up a copy of admiralty chart 5011 for a dive into the colours and symbology used.