Why do tix get more expensive the closer to the flight date?  

I would think they would want to fill the planes and discount them the last day or so.

Anyone hve any real insight into this?

The airlines know that people need to get someplace quickly unexpectedly (as in business travel) and cash in on it.

Supply and demand. As the flight time/date gets closer the supply of empty seats decreases. The remaining demand for the seats becomes fairly inelastic, meaning a much higher price will not drive away those customers.

Airlines use complicated algorithms to analyze supply and demand for every seat on a daily or even hourly basis. Their goal is to get every customer to pay the absolute maximum price they're willing to pay for their seat at the time of booking. That's why you may get 2 different quotes for the same flight if you check the price several hours apart. They don't necessarily need to fill the plane to maximize profits.

I would think it helps with planning, too.  Airlines will cancel flights due to an insufficient number of passengers.  If everybody waited until the last minute to buy a ticket, it would be considerably harder to plan ahead (for staffing, gate availability, etc).

For the same reason a dog licks himself

The only time tix get cheaper close to the flight is if the flight is seriously undersold, and in danger of cancellation (even that takes quite a bit, since it disrupts three- or four-leg aircraft rotation routes).

Ticket aggregators/brokers can get discounts at the last minute, too, to help fill undersold flights.  The airline would rather release tickets to the likes of priceline than sell directly for under-policy prices, as it sets a bad pricing precedent.  When you buy your ticket from an aggregator, it gets a different fare code, can accrue frequent flyer miles differently or not at all, and gets bumped down in terms of upgrade and bump precedence, compared to direct-sold tickets.