Because we are not trees/plants.

a plant doesn't replace anything, it just grows as much as it can. if you think about it, this is why pruning works. lrn2biology

The wiring that plants have to run through things to make them work isn't as finicky and their body plan is a lot more modular.

>>2086959
It's called vegetative growth for a reason.
Also plant tissues are far less complex than animal tissues. Growing a new branch does not "replace" the old one, the plant does not have a defined body plan, but rather a growth habit.
Complexity has advantages and disadvantages. For an organ or limb to function properly it must be the correct design and have all the right connections. Animal growth and differentiation is mostly a one-shot process.
Healing processes focus not on replacement of missing tissue, but rather on preserving the life of remaining tissue.
There are exceptions (lizard tails, salamander limbs, some arthropod limbs) but what it all comes down to is evolution.
Infinite regeneration and life is not a survival trait on the genetic level for most ecological niches.
Adaptation requires reproduction and mutation.
Without playing the genetic lottery with each new batch of children, eventually the environment will change enough that your traits will lead you to death, regardless of regenerating lombs and biological immortality.

If nothing died of old age, each new generation (containing the mutations that might keep the line going) has to compete with all the other generations for resources. Starvation. Extinction.

>>2086959
animals are living organisms
animals can regrow lost limbs

Lizards (animal), jellyfish (animal), octopus (animal), starfish (animal) and a great many other animals can replace limbs.

In general, for a large animal, scale is working against them. The volume of the body, which equates to the number of cells that need to be fed and the nutritional demands, increases faster with size than the surface area and other factors.
This means larger animals are both Weaker relative-to-weight (an ant is only strong because it is so tiny) and have less margin to keep themselves fed when badly injured.
Basic geometry says regrowing 2x scale limb doesn't take require growing 2x as many cells back, it would require 8x as many cells. (1x1x1 = 1 volume, 2x2x2 = 8 volume, holds true any time a trio of dimensions is doubled)
A lion losing any limb would starve to death from inability to catch prey long before the months it would take to regrow any arm or leg (if that were even possible)

Extremely smart social creatures willing to take extremely long and expensive care of the amputee with personal sacrifice is the only situation in which it would work for large animals.
And really, by the time a group becomes so social and sentient that they can care for disabled, it'd be hundreds of thousands or millions of years faster to develop medicine than to evolve limb regeneration.

>is capable of replacing/regenerating a lost limb
they're not actually capable of doing that. what you're saying implies that there's some kind of developmental program for a given limb of a plant - there's not. there's a meristem that has inherent dynamics of division and differentiation that will determine how long it grows and how many organs it initiates and where, but that meristem will behave similar to every single other actively growing meristem on the plant. there's not a program for "first limb, left branch" or something like that. you're not replacing a given limb, you're growing an entirely new, biologically distinct limb in a different place

it's a subtle, nitpicky distinction but it has important biological implications

https://youtu.be/QFa6jP6WgzM

Scishow is good at explaining things