The battery is their own product, or manufactured on their behalf. If there is a failure it's not a simple swap like it would be with a failure of say Bosch or Shimano. Even then I've had friends in both camps wait 3-4 weeks for replacements. One guy had to wait the same for a replacement shock that had to go back to Silverfish. These are large percentages on the profit margin of the bike. Rather like an engine blowing on a car. The motor and battery on an eBike are around the same cost.
I am only speculating but I am guessing if they don't have a simple swap for you in stock it has to go through the QA process from the manufacturer that made the product. They can't pick up a phone and say we need 300 batteries and they arrive next week. Not as things stand.
If you want a same day swap on almost any component we all know where to go and what to buy, but you pay a premium for that and we all don't live near Berkshire. This is still direct to consumer brand that has taken a bold step to have a service centre in the UK. It's not going to be perfect from day one...You aren't dealing via email with Germany now.
Supply chains and the industry have been hammered right now. Speaking to the guys at YT, they are just as annoyed and committed as you are dude. This sucks, but I can assure you having met all the boys there, they are trying the best they can in current times.
Annoyingly, if something is going to fail, it fails fairly early.