Why aren't there any zombie attacks on the base?

The base builder used to include periodic zombie attacks which would affect the defence and morale percentages of your base. These were removed because some players found it discouraging to have their bases damaged and be unable to rebuild if they weren't able to run often enough (especially when this was due to illness or injury). However, we know that many people did enjoy this aspect of the base builder and it's something that we may look at reintroducing in future.