Except most technology made today is not designed for longevity. Tech that requires an internet connection to a working server to function even though that has nothing to do with it's core purpose (see https://mobile.twitter.com/internetofshit)
"Why doesn't the seller pay for one up front and share the results with interested buyers?"
In the Bay Area they do. No one makes offers with an inspection contingency because it's so competitive but often the seller will already have done an inspection that they share with potential buyers
An old Android tablet or phone with the app Dormi does great, too. The best part is that it doesn't make any noise until there's sustained noise detected in the baby room. It also alerts if it loses connectivity and can be snoozed at 10 minute intervals.
A bit of UI design feedback - the single capital letters in the how it works section are confusing. It seemed to imply that "FHC" was a meaningful acronym. Might want to reconsider that choice.