It's not just the burning, many toxic chemicals are already present and the processing[1] makes it worse. The processing converts things like nicotine, and happens at relatively low temperatures- lower than those in vaporizers. Plus, keep in mind that nicotine is in tobacco as a poison for insects. Tobacco doesn't want to be consumed by humans either.

[1] https://en.wikipedia.org/wiki/Tobacco-specific_nitrosamines