Unless you or someone else deleted it from the template.
From out of the box cart.tpl
Code:
{if !$std_checkout_disabled}
<div class="checkout-button">
{include file="customer/buttons/button.tpl" button_title=$lng.lbl_checkout href="cart.php?mode=checkout" additional_button_class="main-button"}
</div>
{/if}
Note sure where $std_checkout_disabled is coming from but the button code is in the template. This could be a setting in admin or std_checkout_disabled means any standard payment method in exception of paypal and/or google or something like this