If the owner has people sitting there and wants them to purchase more, that's more increasing average revenue per user which is limited. How much coffee can someone drink? If someone drinks a plain coffee, getting them to upgrade to a higher margin sugary drink is hard. You can push food and snacks, but that's also limited since you're not a restaurant and the person can only eat so many pastries or dry goods. For every one customer sitting down there are likely 10 customers that come in during that time. Why focus on the person sitting down?
Getting more people in the door is probably an easier way to get more revenue. Especially considering the amount of people that can sit around all day versus the amount of people that can walk by and pick up a cup of coffee to go. In that regard, competition is definitely a huge factor. If a coffee shop has a 15 minute wait while another one is 5 minutes, I would likely pick the faster one. You can prefer one brand to another, but service is a huge factor.
the biggest problem is almost always reaching their target segment--like the people already sitting there with laptops--to drive enough transaction volume to be sustainable, which is squarely a marketing problem. rent and labor costs (operations) are usually the next biggest problems. competition ain't the issue.