Why can't I use a with variable in select?

Question

So I'm trying to calculate the population surround Atlantic Commons.

with atlantic_commons as (
    select geom 
    from nyc_streets 
    where name = 'Atlantic Commons'
),
ac_nearby_pop_per_sqr_meter as (
    select 
    sum(popn_total) / sum(st_area(ncb.geom)) as pop_per_sqr_meter 
    from nyc_census_blocks ncb
    join atlantic_commons ac on 
    ST_DWithin(ncb.geom, ac.geom, 50)
)
select (ac_nearby_pop_per_sqr_meter.pop_per_sqr_meter * 
power(50, 2) * 3.14159) + 
(ac_nearby_pop_per_sqr_meter.pop_per_sqr_meter * 
st_length(ac.geom) * 100) as estimated_pop_in_proximity_to_ac
from nyc_census_blocks ncb
join atlantic_commons ac on ST_DWithin(ncb.geom, ac.geom, 50)
group by ac.geom;

It gives me this error

SQL Error [42P01]: ERROR: missing FROM-clause entry for table "ac_nearby_pop_per_sqr_meter"
  Position: 295

Error position: line: 8 pos: 294

Not sure why I cannot use ac_nearby_pop_per_sqr_meter.pop_per_sqr_meter variable directly in calculation