If you squint you could say that viruses are already doing that. A lipid bilayer is a key component of several kinds of membranes. All of our cells have a lipid bilayers separating the inside from the outside. The corona virus is also built from these same lipids, which is why it's vulnerable to soap.
Anyway, I think the important thing that the other commenter was saying is that mRNA needs to be carefully packaged to be medically useful. You can't inject pure RNA as a vaccine because not only is the mRNA going to be quickly degraded before it gets anywhere but if the immune system sees any RNA floating around by then it kicks itself into a frenzy because free-floating RNA is usually a sign that something nasty is afoot.
They have. That spike protein is part of their envelope ("nanoparticle").
The envelopes used in an RNA vaccine are generally simpler, because they're working under different constraints than viruses. For example, their envelopes don't need to be easily manufactured in a cell.
But some RNA and DNA vaccines do use viruses as their delivery mechanisms, eg the J&J COVID vaccine.
