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Math isn't a science.
Why do you think bio isn't rigorous? Biologists adhere to the scientific method and undergo peer review. A lot of bio depends directly on math, physics, and chem, so if you think bio isn't rigorous then you must either have a problem with how techniques from those fields are applied, or maybe you just have a problem with those fields themselves.
I'll assume your issue is with the application of core math/physics, or their substitution with subfields like statistics. Keep in mind that bio (like chemistry, but more so) is inherently a bit further from math/physics due to greater random variations in experimental settings. Also keep in mind that biology operates with math, physics, and chem as foundations, so it's fundamentally more complicated (complexity increases as your foundation expands).
Biologists have to manage this complexity somehow... this is where statistics can help, especially in controlled experiments. The community also helps get rid of bad science when experiments cannot be replicated.
If you think that use of statistics, or probability, make biology less rigorous, then you should probably recognize that statistics and probability are used in math (ex. in number theory/group theory/cryptography) /physics (see astrophysics)/chem (probabilistic models of location of electrons or something) as well
