Because they announced it as 'the most expensive film ever' when 2.0 was still filming and we already knew Akki's movie was costing 200-250 cr- ERGO Brhahamster was already costing more AT THAT TIME. And that was in 2018.
So, at the time costs were surging to 250+-300 cr. Add whatever they had to put in since , all these additional cameos (I hear SRK is not the only one having cameos)..aditional outdoor shoots.. reshoots.,...additional VFX they claim it's neded.. I'm guessing they are getting to 350-400 cr.
And that's why maybe they shoot in portions. As soon as they get like 20-30 cr..they do some work.
 But each time they need to do such shoots and regroup the crew , costs are there for all- equipment, cast, food, transport..etc etc
7