Kubernetes probes are much different in my opinion.
Your Kubernetes liveness check will check if things are working inside of your cluster which is great for a high frequency checkup to potentially modify the state of your pod based on the result.
But Uptime Robot is an end to end test. It tests a real connection over the internet to your domain which exercises external DNS, traffic flowing through any reverse proxies, your SSL certificate, etc..
Both compliment each other for different use cases.
Fortunately using an email address when registering a new Let's Encrypt cert will let you get warned by LE if something happens where your cert doesn't get renewed and is about to expire.
The reality however is that every managed node (like on GKE) uses quite a lot of CPU and memory out of the box, for which the user pays. On top of that there're cluster fees, just for having it around. This makes it completely unfriendly to hobbyist projects, unless one is ready to pay dozens of $s just to have Kubernetes (prior to deploying any apps to it).
(And sure, there're free tiers here and there, but they never solve this problem completely on any of the big cloud providers, at least)
Compare that to managed "serverless" offerings (even pseudo-compatible with K8s API like Cloud Run), which eliminate the management fees, but impose a tax with latency. Oh well.
Compare this to a serverless offering that is multitenant by design, the control plane is shared making the overhead cost of an extra user is basically zero, which is why they don't charge you a fee like this.
IMO if you're a hobbyist interested in K8s, your best way to go is to install K3s, which is a lightweight, API compatible K8s alternative that runs on a single node. It's pretty nice if you don't care about fault tolerance or High Availability.
At the end of the day, all resources are run by the cloud provider on KVMs sharing the same physical machines anyways, so it's up to them how much to charge. The fact that both small and large customers get to pay for the same amount of resources allocated for them, only means these resources are not allocated in the most efficient manner. So a cloud provider could fix this.
We should also not discount the net positive effect of attracting more hobbyists and startups to your platform. That's how AWS and GCP started, for example, but now they're just focusing on more enterprise business so smaller ones mean less to them (although AWS arguably less so). But we shouldn't forget that while they don't contribute as much to the revenue, they're essentially a free advertising resource that make your platform stay "relevant" (and especially more so for burgeoning startups that could grow to bring more revenue in the future!). The moment they leave, the platform just becomes another IBM that's bound to die, for better or worse.
On top of that, the anti-analogy with serverless for control plane breaks down, because one could always run it on the same shared pool of resources in gVisor or Firecracker, just like with serverless.