(ok, I considder myself more than just a lump of matter.
The difference between life and dead matter is,
that life actively counters a gain in Entrophy by
using metabolism. Dead matter does not, at least not
by some purpose.)

Anyhow. In Evolution species evolve by adaption to an environment.
If the environment is very very static, and there are almost
no natural predetor, a species can optimize to live
an almost immortal life -> at the cost of having to have
a very simple organic structure.

Such is the case with fungi and sponges.

But mammals dont live in such an environment. Also their
bodystructure is much more complex and fragile.
Immortality would come at an energetic cost
that would make the mammal have a hard time to compete against
fast-generation populations.

As I said, long living self repairing tissue is very energy intensive,
and likely to operate very slow (metatobic rate) and
be clunky due to needed heavy protection against environmental factors.
(Giant Turtles went into that niche)

Such a species is not very fit against its short living competitors.
-> who can develop fast growing, powerful organs,
that dont need to be build to last very long.
(Like a Formula 1 car vs a Diesel Traktor)