Let's imagine there only existed one planet in the entire Universe, and it is Earth. Now I would suggest that it would be utterly extraordinary if it just happened to be ideally suited for life. It would be far far more likely that it would be a planet wholly devoid of any life.
But, of course, there are at least trillions
of planets in the Universe. The overwhelming majority are likely to be
hostile to life. So why do we happen to live on one suitable for life?
Well, obviously because we couldn't have evolved on any of the planets
hostile to life!
Now, the Earth is ideally suited for life. Hence, even if we knew of the
existence of no other planets, it would be overwhelmingly likely that zillions of other planets must exist.
This is the precise
same argument whereby we infer there must exist zillions of
other Universes, all with different physical properties. In the
overwhelming majority of such Universes life simply could not arise.
The reason why we live in an incredibly unusual Universe that happens to
permit life is precisely the same reason why we happen to live on a
planet which is ideally suited to life.
The other alternative is
to suppose there is only one Universe. The reason why the physical
constants and properties permit life must be because some outside
influence -- a creator of some description -- constrains the Universe to
be that way.
Obviously scientists prefer the multiverse hypothesis.