Why is scaling out preferred over scaling up?

Scale up - Make an instance bigger, e.g. make a VM bigger

Scale out - Add instances of an application

Since you are adding instances of a service the existing instances are not modified therefore there is no interruption to service. With the scale up often the instances have to be restarted to have the resources added/recognized causing an interruption

By having multiple instances you not only increase scale but add resiliency since if an instance fails the service is still running on the other instances

With scale out the instances can be updated in groups avoiding taking down the entire service during maintenance

Scale out generally costs the same since having two VMs with 2-cores generally is the same cost as one VM with 4-cores (with the exception of some additional storage for the OS disk)