The mechanisms that created the universe are an active area of study, and no scientist is flatly declaring the matter settled. When scientists come up with theories they're based on evidence and testing, not conjecture or folklore.
If the universe did not "start by itself," what are the alternatives? An invisible personage poofed it into existence by magic?
1. Is that reasonable?
2. Is there any tangible evidence of this Personage, or is this guesswork?
3. Is this a "theory" of mechanism, or of agency?
As far as life starting by itself, here we have mountains of evidence of ordinary chemical processes creating and assembling the components of life, using known and tested physical laws. As with the cosmlogical question, what's the alternative?