Why is it illegal to have a phone in your lap?

Likely to make the law in any way practical to enforce. Many people will use their phone in the car by keeping it between their legs like a middle schooler hiding their phone use from their teacher. They can read messages or watch videos while keeping it out of their hands, but it's still just as distracting.

You could just ban looking at a phone in your lap while driving, but then you have the nightmare of proving that someone who glanced down was actually looking at their phone, rather than just randomly glancing down for some other innocent reason. And they would have to glance down at their phone at the exact moment a camera or police officer saw them.

Phone use is actually very hard to enforce because of the nature of its use. People using their phone while driving don't tend to continuously look at the phone the whole time they drive - they would be completely incapable of driving if they did so. Instead, they use it intermittently, such as while stopped at a traffic light or while cruising down the highway. That use is still enough to degrade their driving performance to the level of a drunk driver, but it's not continuous. To make enforcement practical, you need to write the law so that it doesn't require a lucky coincidence to enforce.

For an older comparable example, consider open container laws. You might reasonably ask, "wait, as long as I'm not drinking from it, why can't I have an open beer in the car? Maybe I just want to take my half-finished beer home from the bar and finish it at home!" And while that would be a perfectly innocuous reason to have an open container of alcohol in the car, it would also make drunk driving laws much more difficult to enforce. You could only ticket someone for drinking in the car if they happen to take a sip right when you're watching. Instead of trying to outlaw the infrequent action, you instead outlaw the necessary but continuous action. It's not practical to only ban drinking in vehicles. Instead you ban having an open container, as "possessing an open container" is something a drunk driver will be doing for a protracted period of time.

It's not a perfect approach to writing laws; you do end up criminalizing some innocuous behavior. But trade offs have to be made. Yes, it's unfortunate that open container laws also make it so you can't bring your half-finished drink home from the bar. And yes, it's unfortunate that banning cell phone use while driving also requires banning just having a phone in your lap.

But if you've ever worked in a classroom, you'll know that this is the only way to actually ban cell phone use while driving. Teachers learn very quickly they can't just ban students from using their phones, they have to completely ban them from having them out at all. Relying on lucky coincidences to enforce laws is not a practical solution.