Why do people claim that monarchs in constitutional monarchies like the monarchy of the United Kingdom contain no power when de jure they hold almost all of the power in the country with the country not being abe to legally function without a monarch and de facto still holding powers such as reserve powers and even royal assent to bills? This question is more than just about veto powers. It is about all their powers such as declaring war or making treaties and all their other powers.
The short answer is: even though you are correct that the monarch retains those powers in law, in reality these powers are never exercised.
The monarch formally retains a decent list of powers. However, the number of functional policy or process abilities is small. We have had questions on specific examples such as veto power. The consistent conclusion is that while the monarch has formal (de jure) powers, in a more practical (de facto) sense they have very little. In practice, many of their formal powers are utilized by cabinet members or other policy makers and the monarch symbolically approves them.
This is an interesting case of differences between de facto and de jure abilities. Often times the two coincide (law allows someone to do something, and they can do it). When they are different, we usually imagine a case where the law doesn't explicitly allow something, but it happens anyway. In this case, the law allows for something that can't effectively happen.
Finally, as a kind of parting pedagogical remark, it's important not to over-estimate the value of written documents when thinking about politics. Policies, laws, and constitutions are valuable for certain kinds of questions. However, it is a mistake to imagine we can understand the realities of politics just by reading those things. Politics is bit like ecology - it's defined by the interaction of a lot of moving parts.