Its uncanny that when people think to time travel, they will be drawn to paradoxes. Not to say that we CAN prove it, because the reality we will never know when a paradox occurs. But supposedly to say using time travel to change the past, if we think of time as a book, after umpteen chapters of writing, we remove a supporting character it may not make much of a difference to the reader but supposedly we remove a main character, it would make the book ridiculous so therefore, the writer has 2 options. Continue to read the book, omitting any issue of the erased character wherever he thinks should be included, or write an entirely new book. Therefore it would explain why there are no time travellers visiting us now. Either, they realise that it would not be prudent to go too far back into time to change anything because that would change too many details of the storyline, or only choose to go back a chapter or two where the changes would not be too damaging.
Either way I think the book "the time machine" by H.G. Wells (a good writer btw), has a pretty nice way of explaining time travel.