Why does Windows need to restart every time it updates while Linux doesn't?

Even the smallest update of one package or security patch makes Windows need to restart. Why?

If an anti-virus application doesn't need it after patch update, why Windows need it after updating Security Essentials' virus-signature files?

On the other hand, Linux mostly doesn't need it except when it upgrades its kernel.

Note: I am talking especially about Windows 7. I don't know much about Windows 8, or the Server editions.

Restarts are sometimes a necessary evil when an update installs on windows, simply because the files that need to be updated are in use and can't be modified until the PC reboots. On other hand, a linux system can be upgraded in real time while it is working and still have a hundred percent uptime and all that needs to be done to start using the upgraded program is to restart it. It all comes down to how linux and the file system handles files. When linux runs an executable, it loads the whole file into memory and accesses it from there. This means that there is no connection to the physical file on the disk drive. When the program is closed and all connections to the file are cut, the file is deleted from memory so while the program is running, from memory, the physical file on the disk can be updated or changed at will and the running program is not affected.

Now Microsoft claims that Win 8 OS requires fewer restarts after a patch, bug fix, or other Windows update finishes installing as it will consolidate all restarts for the month to coordinate on the day of Microsoft's familiar Patch Tuesday. This means that any update requiring a restart will wait until the second Tuesday of each month, essentially prompting you to reboot your PC only once a month. Exceptions would be made only in the case of critical security updates that need to be applied immediately.

Windows doesn't ask to reboot every time an update is installed. For example, it doesn't ask to reboot if the definitions for Windows Defender are updated, using Windows Update, or it doesn't ask to reboot when a new device and the related driver is installed/updated. In the latest versions of OS, the restart is strictly required in case of major updates, like security updates for the system or its modules, like updates for .NET Framework.

The reason of the restarts in Windows is that Windows isn't able to update important files while they're in use, because they're locked while the OS is running. When the OS restarts, the files doesn't have a lock and they could be overwritten and updated. The difference with Linux is the different architecture. While Windows locks its important files, Linux doesn't and the files could be updated or deleted in an easier way than Windows. For this reason, Linux asks the user to reboot only few times.