How does low volume explain why ITM call's ask price < its strike price + option premium?

Selling a heavily in-the-money call is effectively a financing transaction - the large option premium is a large cash inflow for the seller, and the risk is very similar to a straight long/short position in the underlying. Market makers do not want to extend financing to other entities via option premia, and so in-the-money options typically trade cheap.

When you are dealing with such comparisons, the first thing to do is to verify that the quotes are from real time because with illiquid options, the option quote is often stale.

Let's assume that the quotes are good. At $9.72, the intrinsic value of the $7.50 call is $2.22. The broker is showing that "they are worth 2.18 each" because they are simply averaging the bid and ask prices. That's not an accurate reflection of actual worth. If the bid/ask briefly changed to $2.00 by $3.00 with no change in underlying price, would the average of $2.50 then be what they are worth? Hardly.

The reality is that ITM illiquid options have wide spreads and the bid usually trades below parity. You could place a STC order for a higher price. Maybe another trader comes along and you get 5, 10, 15 cents of price improvement but there is no incentive for the market maker or anyone else to give you the full intrinsic value. While waiting for a better fill, XYZ could drop in price and you would give back some of your long call's gain.

So yes, if you sell at the discounted bid, you'll take the haircut and the market maker or floor trader who buys at that price will exercise for the arb.

If you don't want to take the haircut and you have the funds in your account, do this Discount Arbitrage yourself, avoiding the haircut. Short the stock to lock in the intrinsic value and then exercise the call. If instead you exercise first, you'll have brief market risk and maybe you do better, maybe you do worse.