One of the main reasons people build raised beds for vegetable gardens instead of planting directly in the ground is because it's often easier to fill a raised bed with a good soil than it is to try and improve poor soil in the ground.
Raised bed are commonly filled with either good topsoil and compost mix or something more like a soilless or a peat-based potting mix. I use a peat based potting mix for a raised bed I do square foot gardening in.
In either case the soil in your raised beds will be nicer than the soil in the ground. Nearby tree, shrub and plant roots may find their way into your raised bed soil and realize how nice it is in there and decide to stick around. They will compete for water and nutrients with the plants you want to put in your raised bed. The landscape fabric will help keep them out but some roots may still find their way in. Landscape fabric doesn't last forever either.
The depth of the soil in the raised bed should prevent weed seeds that are in the existing soil underneath from germinating.
If you have burrowing animals like gophers you may also want to use some hardware cloth on the bottom of your raised bed too.