Tolkien addresses this somewhat in an essay titled "Notes on Motives" (emphasis mine):
Morgoth lost (or exchanged, or transmuted) the greater part of his original 'angelic' powers, of mind and spirit, while gaining a terrible grip upon the physical world. For this reason he had to be fought, mainly by physical force, and enormous material ruin was a probable consequence of any direct combat with him, victorious or otherwise. This is the chief explanation of the constant reluctance of the Valar to come into open battle against Morgoth. Manwë's task and problem was much more difficult than Gandalf's. Sauron's, relatively smaller, power was concentrated; Morgoth's vast power was disseminated. The whole of 'Middle-earth' was Morgoth's Ring, though temporarily his attention was mainly upon the North-west. Unless swiftly successful, War against him might well end in reducing all Middle-earth to chaos, possibly even all Arda. It is easy to say: 'It was the task and function of the Elder King to govern Arda and make it possible for the Children of Eru to live in it unmolested.' But the dilemma of the Valar was this: Arda could only be liberated by a physical battle; but a probable result of such a battle was the irretrievable ruin of Arda.
History of Middle-earth X Morgoth's Ring Part 5: "Myths Transformed" Chapter VII "Notes on motives in the Silmarillion" (ii)
Morgoth's dissemination of his power into the Matter of Arda (which he had been doing basically since the Years of the Lamps; this was not a new problem in the First Age) created an unwinnable scenario for the Valar. Either they destroyed Morgoth, destroying most of Arda in the process, or they let him be and he'd do it himself.
Tolkien further suggests that, from the perspective of the Valar, the Noldor were doing a very useful service, quite effectively keeping Morgoth occupied and contained, and also elevating the race of Men:
If we consider the situation after the escape of Morgoth and the reestablishment of his abode in Middle-earth, we shall see that the heroic Noldor were the best possible weapon with which to keep Morgoth at bay, virtually besieged, and at any rate fully occupied, on the northern fringe of Middle-earth, without provoking him to a frenzy of nihilistic destruction. And in the meanwhile, Men, or the best elements in Mankind, shaking off his shadow, came into contact with a people who had actually seen and experienced the Blessed Realm.
In their association with the warring Eldar Men were raised to their fullest achievable stature, and by the two marriages the transference to them, or infusion into Mankind, of the noblest Elf-strain was accomplished, in readiness for the still distant, but inevitably approaching, days when the Elves would 'fade'.
History of Middle-earth X Morgoth's Ring Part 5: "Myths Transformed" Chapter VII "Notes on motives in the Silmarillion" (iii)
Later in the essay, Tolkien notes that the intervention of the Valar in the War of Wrath was actually a calculated move; Morgoth was severely weakened, and contained to a single region of Arda. He could thus be defeated more easily, and only ruin part of the World, rather than all of it:
The last intervention with physical force by the Valar, ending in the breaking of Thangorodrim, may then be viewed as not in fact reluctant or even unduly delayed, but timed with precision. The intervention came before the annihilation of the Eldar and the Edain. Morgoth though locally triumphant had neglected most of Middle-earth during the war; and by it he had in fact been weakened: in power and prestige (he had lost and failed to recover one of the Silmarils), and above all in mind. He had become absorbed in 'kingship', and though a tyrant of ogre-size and monstrous power, this was a vast fall even from his former wickedness of hate, and his terrible nihilism. He had fallen to like being a tyrant-king with conquered slaves, and vast obedient armies.
The war was successful, and ruin was limited to the small (if beautiful) region of Beleriand.
History of Middle-earth X Morgoth's Ring Part 5: "Myths Transformed" Chapter VII "Notes on motives in the Silmarillion" (iii)
