Why does the Arabian Peninsula have no clear geographic boundary with the Syrian Desert?

The Arabian Peninsula and the Syrian Desert lack a clear geographic boundary because they merge imperceptibly into one another in the north through the Syrian steppe, a treeless plain.12

While there is no distinct natural line of demarcation between these two arid regions, the northern borders of Saudi Arabia and Kuwait are generally used to define the northern limit of the Arabian Peninsula. The transition is further characterized by features like Al-Ḥammād, a stony plain that stretches northward from the Al-Nafūd desert into the Syrian Desert, illustrating the continuous nature of the landscape across these areas.234