I understand where you are coming from, but I think if you use the telerik tools for a while you'll quickly understand "why" you don't want to do everything yourself.
As developers, one of our key limits is time. We need to create a web application that does something and we have x amount of time to get it done. Well, I could spend 80 hours trying to build and debug ASP.NET components that are cross browser compatible and have half of the bells and whistles as telerik's and THEN integrate them into the actual application I'm building. OR I could leverage telerik's amazing controls, support, and active community to get my "basic" advanced tool set and spend more of my time building superior applications.
It all comes down to what you "do". Is your core business building controls that other developers use? If yes, then you probably don't start with a drag and drop toolset (though telerik's controls are MUCH more than drag and drop!). Is your core business building web applications that solve business problems? If yes, then why waste your time reinventing the wheel? Spend your time where you can really add value to the business project, and that's probably not developing advanced UI controls.
Think of telerik's controls as "Bentleys"- very beautiful and full-featured tools that make your life easy. While you could build your own car and it may be fun, it definitely wouldn't be a Bentley (if it would be, you're in the wrong business). =)
My life has been infinitely easier since I began using telerik's controls and my projects have been better than ever. I'd love to help make your life easier, so let me know if you have any concerns about telerik's controls and I'll be happy to discuss them with you.