Why Do Updates Keep Requiring A Re-boot?

in bd is 2010 xp pro sp2, the update service will download /install the updates as it should but sometimes it then checks again and does not download anymore files but stops /starts again then asks for a re-boot to complete the installation, can software vendors not make sure that 99% of things are not 100% sorted before they release programs, unfinished software it's a joke on us who are stupid enough to pay a subscription for it, so by previous standards it will take around 6-8 months before these issues are fixed? by wich time it will be nearly time to release another unfinished product no wonder why bitdefenders name has lost its appeal