Napkin math time. Assuming 30% profit (very achievable for a tech biz that size), thats $90M a founder per year (if they want). I would take that over the risk of trying to get to a $1B sale any day of the week. There aren't very many industries that can achieve something like this. What a time to be alive.
Note - I understand their are so many assumptions here, but I think my point still remains.
And if Walmart had the assumed 30% margins, they probably could be a $2.5 trillion company. Their revenue is more than 2.5 times their almost-$1 trillion competitor.
Facebook bought instragam for $1 billion even though there was no significant revenue or income. But the potential for growth of instagram itself and the photo sharing social media space was immense.
With established companies like walmart in establish spaces like retail, you know what you are getting. With tech unicorns, it's a bit of a gamble.
Look at marketo for a good comp: 21x EV/TTV - http://tomtunguz.com/marketo-vista-ma/