The key thing about passwords is that they should be unique but above all long. It's actually been known for some time that adding special characters and numbers doesn't actually make a password much stronger. What it does is make is more difficult to remember and increase the risk that it gets written down on the proverbial sticky note on the monitor, or that it gives a false sense of security and the temptation to use it on more than one site is greater.
As per the famous XKCD comic, a password composed of four random words of reasonable size is far harder to crack or guess, than say Pa55W0rd!&JD, which would satisfy most sites password requirements, over ten characters, includes upper and lower case, numbers and a symbol or two - so all good right?
As humans are actually very bad at coming up with truly random characters, often these 'super secret' passwords incorporate elements which are meaningful to the user, numbers that are actually significant dates, letters which are initials etc. All of which can easily be determined or worked out from what many, openly post on social media forums!
What the machine generates for you as a password will almost certainly be far stronger and easier to remember than anything you could come up with. However I do agree, that you should have the option to use your own password if you so wish, but it certainly wouldn't be fair to say that this would increase security.
