There's a good reason for that: US does not really need to put backdoors in software to spy. They have the largest tech companies under their legislation and can already force them to disclose information on users. Most companies won't argue and hand over things silently.

They have international agreements with a number of countries for surveillance.

This means that unless an app is completely hosted, in all parts from source code to production, in a privacy-friendly country, on servers provided by a privacy-friendly company and the app is made by a company in similar conditions, the US and EU have all they need.