Because the parliaments were distinct.

European states (especially in the pre-nation state, pre-republic era) are a bit complex and hard to understand.

Parliaments were filled mostly with aristocrats. They had class allegiances, but they also had some identification with the place they lived in.

Monarchs were generally members of families and often had no discernible ethnic heritage (culturally or linguistically) or they had one completely alien to the people of the city they ruled from (many post-1066 monarchs of England until the Hundred Years' War).

They were aristocrats who also had titles to thrones. Imagine a rich family held titles to an important VP role in Apple Computers and one in Toyota that would last forever. Now imagine the same grandson inherited both. Instead of it being split between cousins, only one person has both.

That grandson may want to just merge Apple Computers and Toyota, but the rest of the VPs and middle management at both companies are opposed. (There is no lone CEO or President in this model. The War of Spanish Succession for example, ensured that the absolute monarchy of France would not combine with Spain since an absolute ruler would just merge the two lands as one large crown possession)

You would do your best to steer the two companies into doing partnerships (e.g., iOS installed in Toyota's in-car GPS system) and make sure that neither sued the other over patents. But you would not be so powerful as to simply merge Apple and Toyota.

Apple and Toyota may choose to merge eventually after some prodding, convincing, and a shareholder vote (as happened with England and Scotland becoming Great Britain), but it is not guaranteed.

And East Asia has its fuzzy lines of sovereignty and tributary states. Understanding what a state is and what a monarch is in Europe and understanding what a "Chinese" (as opposed to simply Han) dynasty is and whether East Turkestan existed or was independent or tributary is really hard. European monarchs and European medieval history exist in their own world with their own rules. Even today the monarch of Great Britain (and 6 counties in "Northern Ireland") is a family best thought of as owning paper deeds to Buckingham Palace and many lands in England rented out to Parliament and the State of Britain, while also owning some islands and sitting on other countries' money like a rich CEO owning his/her own island in the South Pacific.

Monarchs in Western Europe (after Western Rome fell) went from tribal chieftains to kings who were just prominent chieftains, to this weird system. Equating a monarch with a throne and/or with a country and/or with an ethnic group and/or with some inalienable homeland is a recipe for confusion. They're orthogonal.

And some thrones were de-unified. For example, one man may inherit the throne his paternal grandfather held and the one his maternal grandmother held, but he may himself only have daughters who in turn had grandsons for him.

One kingdom's Parliament may say that the kingdom's throne goes to the new grandson. The other kingdom may say that that kingdom's throne has to pass through males without skipping a generation. In that case, one kingdom's throne goes to a grandson and the other kingdom goes to some 4th cousin twice removed.

English Parliament even abolished its throne at one point (see the English Civil Wars in the 1600s). Parliament as a collective was a generally equal partner with the throne in ruling a designated place. There was some separation of powers.

The designated place did not even have to be the same.

King Leopold of Belgium owned the African Congo as his personal and stateless tract of land. Only later did the Belgian government seize it and make it a colony of the state of the Kingdom of Belgium. So Leopold was a restricted king in Belgium but ruled over the Congo the same way a rich person rules his yacht on international waters.

Calling England or Scotland at the time you're asking about as "Kingdom of England" and "Kingdom of Scotland" is a bit misleading. It should be "Kingdom and Parliamentship of England". We don't call the USA the "Presidency of America" because the president is not an absolute lone ruler.