That would be because Dropbox is a SaaS software business where the TCO of its infrastructure is absolutely critical to its P&L. Every dollar shaved off its infrastructure per user is a huge deal for Dropbox.
Apple uses software / cloud services as a way of selling expensive hardware, and the margin (if any) made on its cloud services is a rounding error on the overall P&L.
EDIT: Generally, the 'Apple insources everything' meme is a massive oversimplification. Apple insources when there is a competitive advantage to doing so. Apple insources its chip development because it is able to get better thread performance on its phones when the software is aligned tightly to the chipset. Apple insources Cobalt because you need Cobalt to make smartphones and if others find it difficult to source Cobalt, they're going to find it hard to make smartphones, driving up their unit costs. It gets no such advantage from insourcing compute bar a slightly lower TCO, which isn't going to be a huge deal to them anyway.
Apple uses software / cloud services as a way of selling expensive hardware, and the margin (if any) made on its cloud services is a rounding error on the overall P&L.
EDIT: Generally, the 'Apple insources everything' meme is a massive oversimplification. Apple insources when there is a competitive advantage to doing so. Apple insources its chip development because it is able to get better thread performance on its phones when the software is aligned tightly to the chipset. Apple insources Cobalt because you need Cobalt to make smartphones and if others find it difficult to source Cobalt, they're going to find it hard to make smartphones, driving up their unit costs. It gets no such advantage from insourcing compute bar a slightly lower TCO, which isn't going to be a huge deal to them anyway.