Generally speaking for most business types, in-house custom software is a very bad thing because it forces the new owner to also become a software developer rather than being able to focus on their primary business. If you can't call another company to deal with system issues, that means you ARE the company. If you build in-house software, that means there is no company who is responsible for that software to work properly so you are forced to do it.

Even if you give them the phone number to the person who programmed the software originally, what happens if that person doesn't answer the phone or gets hit by a bus?

If you have to hire other people to work on the software, they will be slower because they have to learn the system as they go, so it will probably double the cost and expense of any maintenance or customization work that needs to be done.

Software usually has to be upgraded and maintained every year with operating system or database patches and scripts constantly. Think about how many updates a simple Wordpress website needs every time you open the admin panel. We can click "update" ourselves because it's a personal website but if an entire business with employees and lots of revenue depends on that software to work correctly, it would be very dangerous for us to click that update button ourselves because when you update software, there is a high probably that the update will cause some sort of issue.

When it comes to business software, you want a specialist to perform any work on the system in case there is a problem because the cost of downtime is much higher than the cost of paying a specialist to perform the update.

I can think of tons of reasons why in-house software is bad but I can't think of any reasons why in-house software would actually be good.