If they release the leash and let people book "extra" tickets when they miss a flight, next thing you know people will be taking their S.O. along on conferences using "rescheduled" tickets or having the company pay for vacation tickets and so on. An interesting unofficial benefit, not necessarily a bad idea if it can be controlled. However people always push the boundaries and next thing you know...

So it saves money in the long run to have one employee traveling equals precisely one (admittedly expensive) ticket. They can put their oversight labor into preventing more elaborate forms of control fraud such as using sales meeting budget for dating purposes or padding wedding receptions into corporate meeting budgets or all the nonsense that happens with "company" cars.

Also the labor savings of phoning it in by taking a week to approve tickets means it could be very expensive indeed to have a protocol that permits same day travel if you need to reschedule, all those tickets being approved at considerable labor expense same day just in case you need one once a year. Or paying you for a week to sit there waiting for tickets to arrive, etc.

A side dish is small companies have control fraud oversight as a small rounding error, but when you start thinking about an entire department to organize travel, now you need oversight of an entire department, leading to strange inefficiencies.