For the same reason ISPs and telcos have traditionally included "value-adds" on their Internet connectivity, like email accounts, portals, news, business websit

For the same reason ISPs and telcos have traditionally included "value-adds" on their Internet connectivity, like email accounts, portals, news, business websites, etc, etc, etc, because they don't want to become commodity "dumb pipes". The phone manufacturers need to create and push non-standard (and often broken) app launchers and branded skins in order to differentiate, otherwise they'd just be another Android phone maker, rather than, say, the manufacturer of the Samsung phone.