It works exactly that way. Walmart employees get paid less because supply (everyone is qualified to do this job) outstrips demand and walmart can then just pick the cheapest applicants.

In the tech industry the opposite happens. Demand is higher than supply so you're forced to hire somebody who already has a job. The only way to convince them to switch is by paying them more than they already make.

How so? Isnt this just a typical supply and demand issue?

People usually consume a significant part of their income, so they create more demand.

Also, aggregate demand has more to do with central bank policies than the real economy.

Yes, in each particular occupation, an influx of people willing to work for less might lower salaries in that occupation (compare game programing vs CRUD apps; or magazine journalism vs lawyers), but that doesn't exert downward pressure on everyone's wages.

In addition, if it's easy to start up new competing companies, the employers are forced to pass on the savings.

Of course it does. Care to explain why you think it doesn't?