The truth is.. it is not always the case. Is it desirable? Sometimes. Much depends on your situation and your required outcome.
There are many forms of water available to the home crafter. There is rainwater, tank water, tap water, distilled, deionised and purified. Many of the “free” water options will depend on the area you live in. Do you get acid rain? Acid rain may affect the pH of your soap so should be tested before using. Do you have “hard water? Hard water can cause soap scum to form more readily and make it more challenging to achieve a good lather than soap made with a purified water source.
Tap water can often contain mineral and metal ions from old pipes. That can cause a scum on your lye water and add small amounts of impurities in your leave on products as well as cloud your liquid soap. In extreme cases water from oxidising pipes can collect particles that can cause DOS in soap (Dreaded Orange Spots caused by oxidised oils) having started the oxidisation process in the soap, albeit rare.
The quality of your tap, rain and tank water should be assessed before you take that route, but if they are good quality water sources you can certainly consider them as the water option for your made goods.
Distilled/purified water have their pro’s and cons too. Pre-packaged water is clunky to carry home and adds a little more to the cost of your product but in some cases it is worth it. While you can distil your own water the cost to do so is likely to outweigh simply buying someone else’s labour. It will give you consistent results no matter where you are and is readily available from supermarkets, hardware stores and car supply shops. Distilled water will not be adding any undesirables to your creation and can add some label appeal if promoted to the right crowd.
Deionised water has had the undesirable ions removed from the water so removes the scumming caused by additional ions but does not address any organic contaminants that might be in the water.
The best way to know which water is best for you is to test products side by side and see if you find any variance worthy of choosing one water source over another. You may be well surprised at the outcome, for better, or for worse.