Why train like a triathlete?
By focusing on different muscle groups and movement patterns for each sport, you can achieve a higher level of overall fitness with less chance of an overuse injury.
Why train indoors?
Without all of the outdoor variables like weather, road conditions, hills, etc., we can monitor each individual's power output or pace to optimize the training for every workout.
Can I prepare for an actual triathlon in 5 weeks?
If you are reasonably fit, yes. If you are new to cycling or swimming, it may take longer. But 10 weeks is adequate for almost anybody to prepare for a sprint-distance triathlon.