In business, you get what you pay for. Cheaper hosting might raise more issues that need to by handled by your employees, who also are expensive, and also the organization's focus gets disrupted. The hosting company / cloud vendor has an enormous economic advantage, with access to the entire hardware and software stack, the engineers who built it, people whose full-time job is operating it. Often it's cheaper to pay more for better.

As I have to explain about open source, 'Free is only free if your time is worth nothing.' (And I use a lot of FOSS, it just not always the solution.)

As I understand it, you have to often use multiple gateways based on which one is cheaper and can deliver your message to the recipient, and also take care of handling retries in case one gateway fails. This is not something you typically want to handle if you're not aware of it, and the process of having to talk to each vendor and figure out their limitations is tedious.

There's a lower bound on what these services can charge in the form of interconenction fees charged by the mobile service providers delivering the messages.

In the US, that's effectively zero due to the US phone infrastructure largely using a shared-cost model, but in most other countries which use "sender pays", these fees can be significant.

DO, at least, has bad peering agreements that will cause you noticeable, unfixable (if you stay on DO…) persistent problems at large enough scale.

I use Hetzner, but they have a bad rep for killing services that attract too much attention, e.g. DMCA requests