Why couldn't Rome Conquer Arabia?

"Why couldn't Rome Conquer Arabia?

At the height of its power, the mighty Roman Empire controlled a massive 5 million square kilometers.
From Spain to Egypt, the Romans had demolished any armies brave enough to stand in their way. And yet, throughout its rule, there remained a glaring blank space on its map. It was the place that the Romans just couldn’t conquer – Arabia…

For the Romans, Arabia was the key to an endless supply of lavish riches – silks and spices, gold and silver.
To rule Arabia was to open up trade with the Far East, and from there – who knew? They simply had to have it. "