because we’re dealing with FCFs or cash flow.
for the purpose of calculating FCFs, we’re trying to see where cash comes and goes.
an increase in CL means we have more cash (we defer payments for later date)
an increase in non-cash CA means we use up cash to buy those assets.
notes payables is excluded because it’s considered debt and is handled in net borrowing (relevant for the calculation of FCFE and not FCFF)
unfortunately, another use of working capital is to measure liquidity of the company and for this purpose, cash is included in WC.
if you have trouble understanding this, imagine a simple distributor business who buys inventory and sells them.
buy inventory at credit, AP goes up, Inventory goes up. net change in working capital (excluding cash) = 0, no effect on FCF.
buy inventory with cash, inventory goes up. net change in working capital (excluding cash) = positive, which reduces FCF. this is consistent because we use up cash.
sell inventory at credit, inventory goes down, AR goes up. net change in working capital (excluding cash) = 0, no effect on FCF
sell inventory for cash, inventory goes down. net change in working capital (excluding cash) = negative, which increases FCF. this is again consistent because we just received a cash inflow
if we include cash in the working capital adjustments, we can double count cash inflows and outflows.