Doesn’t it make more sense to keep them in the womb until they have functioning senses?
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Evolution has only one rule: whoever dies with the most grandchildren wins. Note that I say “grandchildren” here: it’s not enough just to have a bunch of kids unless many of them survive to have their own children.
There are a number of ways to achieve this, with respect to pregnancy, and different species have stumbled upon different routes. You’re right that longer pregnancies would mean more functional newborns, but it would also mean larger babies, which are harder for the mother to carry and more dangerous during the birth process, *plus* you can’t carry as many of them. Some creatures, like primates and equines, found that tradeoff to be worthwhile. Others, like rabbits and cats, didn’t. It’s just different routes to the same goal.
There could be many factors here.
How much nutrients can the mother provide via the placenta? Blood sugar is toxic in high concentrations, and larger fetuses require more energy to sustain – perhaps more than is healthy for the mother.
The birth canal can only facilitate so large of offspring. Having growth continue outside the body may facilitate other survival features of the species (hips that are optimized for sprinting?)
For mammals, nurturing with milk may provide better infant mortality at this particular stage.
It’s all trade-offs. Evolution finds a more or less optimum solution for each species.
Sure, it would be great (for your genes) if your offspring were born fully grown and independent. But this would require a very long gestation period and would be a lot of weight and volume for the mother to carry around and give birth to.
The priorities and strategies differ between species. For example for flying birds it would be completely impossible to carry around multiple well developed offspring. For species like horses it’s apparently important that the newborn can more or less immediately follow the herd and keep up. Of course the disadvantage is that the pregnant horse has to carry around all that weight for a whole year and puts her at a higher risk. Compare that to tigers which only have ~100 days gestation period and give birth to two to four offspring. Tigers can also get pregnant again within the same year if the first litter dies.
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