This is the same as the gun laws in the US, which they reject.
Why did some governments decriminalize prostitution rather than rescuing women from prostitution and rehabilitating them?
And, if prostitution is legal in those countries, why are US-style gun laws not adopted?
You appear to be assuming that prostitution is inherently wrong and horrible, and wondering why anyone would enact laws to allow a wrong and horrible practice.
The simple fact is, not everyone agrees with you. In some countries enough people are of the view that consensual sex work is perfectly fine that the laws are set up to allow it. Where difference of opinion exists it's not surprising that different countries have different laws.
I'm honestly not sure exactly what you're trying to get at with the comparison with US gun laws. Is it that you also think permissive gun ownership laws are inherently wrong also, and if a society allows one inherently wrong thing it should be expected to allow others? I think you will find that a controversial way to frame the legal situation in the US! I genuinely don't get the connection between these two issues that would lead you to expect laws about them to be correlated.
There is one sensible comparison I see, but it doesn't argue for your point at all. Some people see widespread ownership of guns as a bad thing for society, and countries where those people are (or historically have been) a majority tend to have more restrictive gun ownership laws. But other people think that widespread ownership of guns is acceptable or even a good thing (like the arguments that the population of the US ought to be armed enough that they can deter the government from tyranny by threat of violent revolution). So in countries where these views are (or historically have been) in the majority the laws tend to be much more permissive of gun ownership. Different countries have different laws on gun ownership, based on the different views (and histories of views) of their legislators and/or voters.
But this is exactly the same as the situation with prostitution. Some people see prostitution as a bad thing, and others see it as perfectly allowable (others see it as bad, but not bad enough to actually ban; the spectrum of views is far more complicated than I am covering here). Different countries have different populations with different majority views. If you think the topic of prostitution laws is like the topic of gun ownership laws, why would you be surprised by different countries having different laws there?
Just to be clear, people who think prostitution is an acceptable form of work are talking about consensual prostitution. Everyone accepts that being forced into prostitution is a terrible crime. But outlawing consensual prostitution is a distinct issue - we don't outlaw every potentially unpleasant job that people might be forced to do, we outlaw the forcing. (Some people do make the argument that it's better to outlaw all prostitution to be extra sure we can catch any case of forced prostitution, more so than e.g. forced cleaning or forced mine work. But again, not everybody agrees with this position, so it doesn't dictate the laws of every single country. Plenty of people subscribe to the opposite argument that making prostitution inherently criminal prevents prostitutes from reporting crimes against them, making it easier for forced prostitution to exist.)
Believe it or nor there are plenty of people who do sex work perfectly willingly and enjoy their work.
