They are simply not designed for the hostile marine environment with constant vibrations and momentum from the boat and of course they’re not designed for salty air. Laptops also consume large amounts of power and often you will need an inverter or adaptor to connect to the boat’s DC supply which introduces more losses and electrical noise. It’s a much neater solution to have a dedicated PC and display both in terms of functionality and reliability. With some simple engineering, you can install a monitor to swivel between chart table and saloon so it can become an entertainment as well as a navigation device.
The Aqua range of PCs from Digital Yacht are designed to be permanently installed and can connect direct to the boats DC electrical system. They consume minimal power and are completely solid state with no moving parts. Despite their impressive performance they are as affordable as a good quality laptop and can support multiple monitors.