At Google's scale, unless you mine bitcoins and actually use CPU/GPU time, the money Google actually spends on your tiny $300 free trial is maybe worth a dollar or two (and if they run free trials on machines that are paid off in terms of taxes, the only "expenses" are a tiny tiny sliver of maintenance - at Google scale network connectivity is basically free).

Cloud providers, especially for 24/7 running services, are a total rip-off even compared to managed servers, and deploying your own bare metal will blow any cloud calculation away when you're getting serious (> 200GB RAM, 50+ cores). The real strength of cloud is when you need extremely high computing power for very short time (e.g. heavy software builds, occasional transcoding, ML training).

And it's opportunity cost we're looking at here. It may only cost them a small amount to provide the resources (although that ignores the cost of developing/maintaining the services) but if they provide $300 of credit to one party that's resources they're not selling for $300 to someone else.