This isnāt going to be a comprehensive answer to your question and may even stray in to a ābrain dumpā on my part, but bear with me.
Can this process be automated? Yes, undoubtedly. No question. It surely is or will be. Back in a former role in 2002 I personally set up about 700 stocks in a single day on to a trading platform I worked for even back then!
Your question really is āwhy so complexā? (and therefore why so, letās say, āslowā?)
OK, here we go.
Regulation in the UK and worldwide markets are messy. Making sure you comply as a startup in a highly regulated environment is paramount. So there are key data points you need to assess:
Where is it traded? Is it legally saleable to my users - all, some, by nation, by expertise? How to distinguish? Is the pre-sale documentation in place? Can it be held in, say, an ISA?
Do I have ongoing data coverage for it? Does my pricing provider feed me daily and closing prices? Do I have ongoing corporate action coverage on this stock in case, say, itās about to reverse stock split and my business will be left exposed if I donāt process this corporate event before market open? Do I have a market making network that can facilitate trading this stock? Is the spread wild and my users freak out when the price paid bears no resemblance to the ālast priceā quoted in my app? What tier of an exchange is it traded on - this affects my ISAāability and often affects my liquidity as, for example, SETSqx stocks can often execute differently to , says, SETS stocks. Where am I routing this order to? When I trade a US stock does it go a different route to market than UK stocks? In the event of a UK stock is it settleable in CREST if I choose to settle in that manner? If it is not Crestable - where do I route the order? Do I have a pretty logo for it? A quirky narrative?
None of this is more than a massive data exercise. Data exercises can be automated.
So then weāre down to supply/demand. Trading this stuff comes at a cost. Human, data and systems. Setting up a stock that 1 person trades once and never again is likely to be low priority. Thatās just economic sense. The āeconomicsā will change over time - so youāll see more stocks added as the company evolves.
Data management in the back office is often overlooked as itās considered somewhat easy. Yet itās the backbone that drives the offering you present for your clients and saves you from regulatory errors (and fines or bad PR) going forward.
Simply adding a stock is easy. Maintaining your compliance and keeping customers happy in the longer term is a balancing act of time, resource and a realistation that things are usually more complex than they initially seem and a combination of human endeavour and tech will be needed to keep the wheels turning.
Spare a thought for your ops teams.
I need to lay down. 
That probably didnāt answer a thing. 
Shorter answer: Can it be automated? Yes. Jāsus, I did it in 2002 in Excel. You need data and brainpower.
