yes u r right.
u may use all the tools from a single EDA vendor(if he supports all the tools in the flow) but the problem is how efficient is each tool in part of the flow depends...
For example
when u do simulations u have cadence nc sim which is faster in case of netlist simulations compared to any othere simulator.
U have primetime from synopsys which is better than the cadence counterpart as it is an industry accepted sign off tool for front end part of the flow.
so ppl use different tools at each part of the flow depending on the efficiency of the tool and its features required and ofcourse the cost!!!!!
Regards
Srinivas