What are your excuses?
There are a million clone apps on the marketplace for any successful game, making 2c per view ad revenue. I don't want to succumb to that model of income stream any more than I want to be a passive income landholder invester/landlord. So this goes to the second reason I dont have my own company: It can feel like 'rent-seeking' when you live in the OSI world, and then chose to use the free stacks, free tools, free cloud, to develop a product you then make others pay for.
Ie "motivations differ"
The answer is simple and uninformative. Because it is the sane/rational thing to do.
It's like asking anyone with a career why they haven't started a business in their field.
It makes a lot of assumptions, ignores a lot of constraints, and treats all aspects of executing as trivial.
Right now, I have two years of experience in trying to do this. And I can sum up my experience as reality is an unpredictable bitch. I'm amazed there are as many crazy people trying to do this as there are.
The only reason I did leap is that I was frustrated and spent more than 10 years waiting for someone else to write the software I wanted. Now, after two years, I see I wasn't wrong, but even with a clear vision, this turned out to be 10x as difficult an area to make progress in than I had anticipated.
As a software engineer, you can ignore almost all of that. Life seems simple, but that's only because you have a literal army of people doing everything else.
Marketing is a huge initial investment to make citing the sheer oversaturation of ads and scams on the Internet as well. There is no guarantee that even paid ads will reach relevant customers because advertising is so gamified everywhere. If you don't already have customers or contracts in tow when starting a business, you're marketing to strangers, who in turn can be scammers.
I literally get calls and messages off of Google that are from fake customers regularly, they even follow through screening calls for reasons unknown and then try to dictate payment terms, which usually begin with me doing work for them and then receiving payment later.
I actually have missed potential clients by mistaking them for scam callers.
You can have the best product or service in the world now and easily sit unnoticed. Success is at the mercy of how much cash you can burn on promotion and development, for years, before you can actually succeed. Even those with solid funding are not willing to risk it on an idea without serious momentum already behind it because of the current state of online scams.
It's easy to create an app, but it's not easy to create an app that sells continuously. There are lots of things other than development too like figuring out better solutions, sales cycles, customer support, etc etc.