I work in insurance and can honestly say about 40% of the people I speak to on a daily basis have failed to disclose something, the vast majority of the time it's purely innocent.
I also know that with a lot of big private insurers declaring mods is an instant decline. For some people it's a simple case of calling the insurer and paying a small fee to add mods but if you're a younger driver who already has limited options if the few company's that are competitive for you decline mods all together you need to find a policy with someone who covers mods and they may well be way more expensive, this is why some people don't bother disclosing them, it vastly narrows down their options.
I'm old enough to know better myself but young enough to understand why people do it.