I think the other answer have valid points; I would like to condense those with my point of view.
Therefore, why would I want any more?
If you are playing competitively, you don't. That is, if you are planning to participate on tournaments or other form of official competition having 40 cards is the best you can have. This is because, as you already noticed, you have higher chances of pulling out the cards you need.
We can see that
1/40 = 0.025, while
1/60 = 0.01666, so definitely you have better chances if you keep your deck as lean as possible. And beyond the fact that you have higher chances of pulling any card, you also have higher chances of pulling the card you actually need the moment you need it (like that Mystical Space Typhoon to destroy that nasty Vanity's Emptiness). Having more than 40 cards only reduces the chances of pulling the one needed, and can make your deck run much slower... well, unless you believe in the Heart of Cards like the Pharaoh did, lol.
Evidence that back this up is the fact that most competitive decks focus on a 40-card build (and if they could use less I am sure they would). It is true that some competitive decks have used more cards; there are cases where some decks ran on a aprox. 42-card build.
There was even one case of a Mermail build that had 53 cards (and even won a YCS), and it worked mainly because of the hability to search themselves that Mermail decks have (mostly because Atlantean Dragoons). But still, this was in 2013 where the rulings and format was way different (slower), and even though it won a YCS I think it was not a brilliant idea to include 53 cards in it (I mean, it's not like we are playing Commander in MTG).
So, it seems clear that it is better to have 40 cards in your deck whenever possible. I would like to complement this with another trend that happened in YGO a while ago.
Meet Upstart Gobling:
Draw 1 card, then your opponent gains 1000 Life Points.
I am sure we have all seen this card at some point, and wondered "Whoa, why would I give my Opponent 1000 LP just for one card"... Well we think this now because LP have become more than just a metric in the game, and have become a resource (several cards require LP as cost for activation, like Soul Charge, and others).
So with this in mind 1000 LP for one card means 1000 extra LP your opponent can spend for an extra monster with Soul Charge... not so great bargain now isn't it?
However, in the past, when LP were not as resourceful as they are now, one could see the benefit on giving "just" 1000 LP to your opponent for a card that could well make him lose more than 1000 LP. This is sometimes referred as the Upstart Theory.
This goes beyond the fact that LP's were not as useful as they are now. If we do our math again, if we include 3 copies of Upstart Goblin we are technically making our deck a 37-card build. Again,
1/37 = 0.027027 which gives us greater chances than before of pulling the card we seek. In competitive games, a simple
0.002 difference can be the decisive factor between winning and losing.
We can see the tactical power Upstart Goblin can have in a deck. That was one of the main reasons why the card got limited to 1 copy. If we go a bit further, we can understand also why cards like Pot of Greed and Graceful Charity are forbidden, as they drastically changed the probability ratios of deck builds.
tl;dr: You don't want more than 40 cards in your deck if you plan to play competitively. If it were possible to have less than 40 cards it would be even more recommended.
Now, if you don't mind about competitive play, feel free to include as many cards as you want up to 60, but expect a slower gameplay and more dead-draws.