Having bomb shelters in every building is not a normal thing that countries generally require. You might equally well ask why Paraguay or Denmark do not have a law to build bomb shelters in every building, despite having the resources to do so. Not having bomb shelters in every single building is not some strange phenomenon that calls for an explanation. Rather, it is the normal state affairs.
Israeli buildings are required to have bomb shelters because Israel has a general policy of being ready for an attack against it by its neighbors. The law has been in force since 1970.
In contrast, Gaza (like the vast majority of countries other than Israel) did not have an extensive history of being afraid that its neighbors will start launching missiles at it prior to the 2014 conflict, so presumably it felt no pressing need for such law, until perhaps quite recently (again, like the vast majority of countries other than Israel).
Even if, hypothetically speaking, they did institute an obligation to build bomb shelters in new buildings in the aftermath of the 2014 conflict, in the expectation that Israel might bomb them again in the future, it would only apply to buildings built in the last less than 10 years, and therefore one would expect that most buildings would not be equipped with bomb shelters anyway.
The only way the premise of your question (that the normal, expected state of affairs would be for Gaza to have bomb shelters in every building) makes sense is if we assume that, like Israel was in the 1970's, Gaza is under constant existential threat from Israel. If you are, in effect, saying that Israel is for Gaza what Egypt and Syria were for Israel in the 1970's and what Russia is for Ukraine these days (Ukraine is another country which recently instituted an obligation to build bomb shelters in every new building), then sure, the premise of your question makes sense. But somehow, I sense that this is not a claim that you wish to make.
Edit: The intention behind the question seems to be to show that Hamas does not care about civilian lives. The argument seems to be:
"If Hamas cared about civilian lives, they would build bomb shelters in every building like we have in Israel. Because they did not do so, they do not care about civilian lives."
This is a spurious argument, not because the conclusion is false (it isn't: Hamas very obviously does not care about civilian lives), but because what is false (as illustrated by most states around the world, with a few exceptions) is the idea that if a government cares about civilian lives, they will build bomb shelters in every residential building.
Overall, using this as some kind of "gotcha" against Hamas is a bit like trying to demonstrate that the Islamic State does not care about human lives by arguing that if they did care about human lives, they would have invested more energy into enforcing mandatory seat belt laws. In other words, you are arguing for a well-established conclusion using a strange and somewhat dubious argument.