Making tunnels that are large enough and safe enough to transport people is a much bigger and more expensive endeavor than making ~1m diameter tubes for transporting goods.
A tunnel would need to be large enough to be maintained by humans when all the pallets crashed into each other and someone had to unjam it. We'd have a new kind of plumber
is as good a place as any to start. It also has plenty of citations of its own if you want to dig further into any aspect.
Basically. the biggest cost of building a tunnel is excavation cost and the cross sectional area of a tunnel is the biggest factor affecting excavation cost for any given ground condition.
Secondly once you add people you have to add emergency escape tunnels and related systems which is just more tunnel that needs digging.
Much of the cost of the subway is the access caverns, life safety systems, etc. For example, NYC built a 2-mile water tunnel for $250M [1]. A subway tunnel costs in the ballpark of $500M-$3B per mile[2].
