If we assume big software houses know how to get top notch developers (as in zero bugs programmer) we can deduct that Microsoft's software must be without bugs. Yet we know that's far from the truth.
The develop their software and when they reach certain level of low priority bugs they simply release the product and solve those later.
Unless you're developing something more complex than a simple calculator, It's not possible to avoid bugs all together. Hell even NASA has redundancy on their vehicles and bugs as well. Although they have much rigorous testing to avoid catastrophic failures. But nonetheless even they have bugs in their software.
Bugs are inevitable just as is human nature to err.
Having no bugs is like having a 100% secure system. If a system is 100% secure it definitely isn't useful any more (it probably lies inside tons and tons of concrete and isn't connected to the outside at all. Not wired nor wireless. So just as there are no perfectly secure system, there's no complex bug-less system.