They aren't even doing that. They are making assumptions about what might have happened.
Let's make this simple. If I find human shaped footprints in the snow outside my house, I can safely assume that a human walked in the snow. Now, there is a slight chance that a monkey walked in the snow, since monkey feet and human feet are kinda similar, but monkeys rarely take more than a few steps without putting their hands on the ground, so, chances are the steps are human. There is also an even smaller chance that some humanoid alien thing walked in the snow, or that a hovering man deliberately put footprints in the snow to trick me into thinking that a human walked in the snow. But it's sort of ridiculous. We've never observed an alien humanoid, nor a hovering man.
Evolutionary science is looking at the evidence and forming a theory. Scientists aren't usually closed-minded to new theories. If you have one that fits better than the man walking in the snow, and can evidence it in any way, back it up, form logical ideas about it, then by all means, present it.
At present, evolution is the stand-out theory for how life progressed on Earth, because the evidence supporting it is almost overwhelming.
Take one look at genetic code, always made up of the same four chemicals, with animals having HUGE streams of code that are almost identical to other animals, and evolution becomes more plausible. Take into account the study at cellular levels, of bacteria mutating and going through generations in a matter of years, and the actual process of mutation becomes more evident. Take a look at the ideas of human mutations, like how, in procreation, a parent with the sickle cell gene can give their child a massively increased resistance to malaria, or how the human brain has evolved to bring conscious awareness, and the ideas for how natural selection works become more evident.
Look at fossil records and you can see bridges between MANY species. Study the reasons for certain anatomical traits and you see how creatures adapt to their environments to survive, and the idea of need-for-adaptation becomes more evident.
Study the development from gills, to oral hinge, to inner ear bones, and you see a fantastic picture being painted of how humans share commonalities with primitive fish and amphibians, and the longer term evolutionary image can become more apparent.
You have to actually study it and understand it before you criticize it.
On a side-note, the idea of intelligent design doesn't have to contradict evolution. Evolution is only the study of how life progressed once it came into being. Evolutionary theory doesn't have anything to do with abiogenesis. Abiogenesis is the theory of how life came to be. Evolutionary theory is the study of the advancement of life once it already existed.
There's no reason that God couldn't have made the laws of physics of the world he created, and let them run their course. You should look at Hebrew. Their word for 'day' can mean 'age'. The world made in seven 'ages' as opposed to days.
If you understood a bit about how the density of matter effects time you could bridge some of the gaps there too. taking into account the fact that energy cannot be created or destroyed, it's natural to ask 'so where did it come from?'
To be frank, the idea that God created the energy that currently makes up our universe is about as good an explanation as any we currently have. All the current theories of what came BEFORE the big bang, from the inflationary universe idea to the ideas of intelligent design, are based on what are inherently assumptions.
The truth is, we just don't know where that energy came from, or whether it simply 'existed' always. In my eyes, time seems to be a big factor. If energy is absolutely dense, compressed and stationary, without motion nor changing forces, then there can be no time as we know it.
It's very, very interesting And there are now scientists, studying the fluctuations of the universal expansion, who contradict other observations and say our universe is expanding FASTER.. That in and of itself may lead to disproving some of the current ideas of the big-bang.
Read it up. It's mind-bending.
