"This is sturdy."
Informal speech is covered later in the course.
Someone learning a new language it can be assumed that the majority of learners probably aren't going to be using it to talk to people they are close friends or family with it. Most will likely be using it with complete strangers or in business settings, so the polite form that they are more likely to use is taught first.
The polite form of verbs are also easier to conjugate than the casual forms so it is easier for beginners to understand before going into more complicated forms.
The casual form of answers are still accepted on most translating questions.
The only thing here that would change it from polite to casual would be changing です to だ