The complex DNS products exist for a reason. For one they can do really good geo-routing. This makes your services go faster for a global audience.
Then, some of the big companies use multiple CDN's. You might want to use one CDN provider in Asia and another in Europe. Furthermore, you may want to select CDN not only on geo-routing dimension, but on arbitrary criteria. Imagine that you had a fixed budget for, say, cloufront, and wanted to route to them as much as you could, but never exceed your budget. Modern DNS services allow all this complex of scenarios.
Furthermore, running your own DNS infrastructure is far from obvious these days. In May 2015 I gave this talk on defending DNS from DDoS:
Draw your own conclusions, but I'd say that running your own DNS makes you _more_ exposed to DDoS and extortion than using someone else's DNS infrastructure.
Then, some of the big companies use multiple CDN's. You might want to use one CDN provider in Asia and another in Europe. Furthermore, you may want to select CDN not only on geo-routing dimension, but on arbitrary criteria. Imagine that you had a fixed budget for, say, cloufront, and wanted to route to them as much as you could, but never exceed your budget. Modern DNS services allow all this complex of scenarios.
Furthermore, running your own DNS infrastructure is far from obvious these days. In May 2015 I gave this talk on defending DNS from DDoS:
https://idea.popcount.org/2015-05-11-oarc---dealing-with-dns...
Draw your own conclusions, but I'd say that running your own DNS makes you _more_ exposed to DDoS and extortion than using someone else's DNS infrastructure.