Demands are high for computing in industry. Industrial machines have to cope with many hostile elements whilst also providing reliable computing. Traditionally industrial computers have been specific, bespoke machines. These industrial computers tend to run old hardware and versions of software. Whilst this has the advantage of making the machines stable and reliable, the out-dated technology may mean the processes controlled by the machine are not done as efficiently.
Many industrial environments are dusty, dirty and grimy. Often temperatures can exceed recommendations for most computer equipment (especially if the industrial environment contains a furnace or freezer where operating conditions could be described as extreme) and fork lift trucks and pallet trucks are zipping in and out of aisles.
If an industrial area is dusty but there are less of the other hazards that have been mentioned, then an inexpensive solution is to use a conventional office PC fitted with protective dust covers. Although not designed for environments with thick dust and grime, for areas with just a moderate dusty atmosphere they may add sufficient protection and can protect keyboards from dust and fluids, preventing sticky keys (and many an unintended typo).
You can, but it will cost you more. Once you've paid for the specialist materials and tools needed, factored in the labour costs and the time it takes to design and develop a unit to your satisfaction, plus the IP certification costs, producing your own doesn't make commercial sense.
The trick is to house the conventional IT equipment in an industrial computer enclosure. These protective cabinets for computers allow the safe use of standard IT equipment in industrial areas as the industrial computer enclosure protects against all the harmful elements - they are waterproof, dustproof, contain additional heating and cooling systems and mostly they are built in solid or stainless steel (ideal for food production) and even when the combined cost of the enclosure and the standard PC is taken into account - they are often far more cost effective than standard industrial computer systems.