They can. There are framework to address this issue and be cross-platform such as React or Ionic. It is very possible.

Very very few mobile developers are true experts in both native platforms. Some can be really good at both - but you'll still find specialists who are better at either. That's why often startups will build a hybrid app which allows them to go live in both iOS and Android app stores, but maintain a single web code base that basically opens in an embedded browser within the app.

yes we can develop fully responsive apps both on ios and android