They have no interest in any kind of cross-device compatibility beyond how it might serve you ads better/faster.
It's not just about serving the ads. The data you generate by using their devices and carrying them around with you is valuable to them. They present value to advertisers by being able to effectively target ads to very specific demographics. To that end, it behooves them to make a cohesive ecosystem which makes people more likely to use more devices more often.
Good MT performance is contingent on good ST performance. Doesn't really matter how many threads you have if they're slow individually. Which will dig a hole faster -- 64 toddlers with gardening spades, or one excavator?