i run this query to get a snapshot of bufferpools:
db2 "select substr(BP_NAME,1,14) bp_name ,TOTAL_LOGICAL_READS total_log_reads, TOTAL_PHYSICAL_READS total_phys_reads,TOTAL_HIT_RATIO_PERCENT tothitratio ,substr(to_char(SNAPSHOT_TIMESTAMP,'YYYY-MM-DD-HH24:MIS'),1,19) snapshot_timestamp from SYSIBMADM.BP_HITRATIO"
result:
BP_NAME TOTAL_LOG_READS TOTAL_PHYS_READS TOTHITRATIO SNAPSHOT_TIMESTAMP
-------------- -------------------- -------------------- ----------- -------------------
IBMDEFAULTBP 35575383 9683846 72.77 2010-01-13-10:10:34
BP_SYSCATSPC 882 0 100.00 2010-01-13-10:10:34
BP_TBS_PROD 0 0 - 2010-01-13-10:10:34

However when I reset monitor using 'reset monitor all' command, it does not get reset:
P_NAME TOTAL_LOG_READS TOTAL_PHYS_READS TOTHITRATIO SNAPSHOT_TIMESTAMP
-------------- -------------------- -------------------- ----------- -------------------
IBMDEFAULTBP 52455565 22155918 57.76 2010-01-13-10:44:19
BP_SYSCATSPC 894 0 100.00 2010-01-13-10:44:19
BP_TBS_PROD 0 0 - 2010-01-13-10:44:19

Turning bufferpool monitor OFF and then ON though does set it to 0.
db2 update monitor switches using bufferpool off
db2 update monitor switches using bufferpool on

Anyone knows why?

Updating the monitor switches is only confined to that session. So if you wanted to see zeros after turning the monitor switches OFF and ON, you need to consider looking the values of logical reads and physical reads from the bufferpool snapshot taken in that session. As the system tables are not confined to that session, you cannot see zeros in the system tables. If you want the system table values to get reset, you need to consider bouncing the instance or updating the database manager cfg for DFT_MON_BUFPOOL to OFF and ON.