Converting small multi-family to central A/C

[Ben Walhood]

In my area I find a lot of 6-flats/6-plexes that do not have central air, despite the fact that it is a feature which is highly attractive for prospective tenants.
When I try to search the web, including BP, it seems that almost no one talks about putting in the money and effort to install central air in these units. However, when I looked up the cost of a wall-mount furnace and A/C combo they only cost about $2k. I realize there's a fair amount of install cost for mounting the unit, wiring the unit, and installing ductwork, but it seems like it could be done for a ballpark of $4k-$5k per unit (I'm using a 2 bed/1 bath with about 750 sq ft as my example). And in my area, it could be argued that rents could go up $50/mth with this feature (possibly more). Not to mention the fact that these units are currently on baseboard heat with the landlord paying for gas (no separate meters). So the switch to electric heating and cooling would significantly lower the building's utility expenses.
Here are my estimated numbers:
$5,000 capital improvement
$50/mth or $600/year in additional cashflow
$50/mth or $600/year in reduced gas bill
Cash-on-cash return of 24% ($1200/$5000)
Appreciation based on 8% cap rate is an additional $15k

My question is this: Why is no one making this upgrade/conversion? Is there a big factor that I'm missing?

Thanks for your insights on the topic!