This is becasue they don't bother to maintain their buses!
Its not because the bus companies do not want to maintain their
buses. Look at it this way, there has been so many buses added due
to BSEP, but the number of full fledged depots responsible for
maintenance remains the same. Thats 5* for SBS and 3 for TIBS/SMRT,
hasn't changed since TIBS opened their Woodlands depot (SBS slbp
replaced jrdep and therefore doesn't affect the numbers).
And LTA built 2 more depots with maintenance facilities but for
some odd reason, the 2 incumbent bus companies are unable to fully
use the 2 new depots, even for a short while before the handover.
Maintenance capacity has therefore remained the same, even if you
can get more technicians, you don't have an increase in workshop
space. LTA could had solved it but they prefer to wait for their
pet project GCM to take effect first.