Hello Samit
It is true that for eBGP, the update timer is set to 30 by default and the keepalive is set to 60 seconds by default. It would be possible to have BGP reset the holddown timer every time an update was received as well, because if an update is received, then the neighbour must be up and running, right?
BGP has been designed so that the update and keepalives are two separate mechanisms. This is because you may want to adjust the keepalives to 15 seconds, the holddown to 45 and the updates to 60. If only the updates functioned as keepalives, you wouldn’t be able to do that. Keeping them separate gives you more control over the timers and how BGP will function.
BGP however is smart enough to send keepalives and holddown messages together within the same packet if they happen to be sent at the same time. This saves on bandwidth and resources in general.
I hope this has been helpful!
Laz