Magento 2 - Set custom attribute value on cart page

No need to create any Observe, you can get it by simple below code.

Just need to create {MODULE_NAME}/etc/catalog_attributes.xml with below content:

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Catalog:etc/catalog_attributes.xsd">
    <group name="quote_item">
        <attribute name="custom_attribute"/>
    </group>
</config>

You can get custome attribute in /templates/cart/item/default.phtml with below code.

echo $_item->getProduct()->getData('custom_attribute');

In Magento 2

Change the following code:

Namespace/Modulename/Observer/salesQuoteItemSetCustomAttribute.php

<?php
namespace Namespace\Modulename\Observer;

use Magento\Framework\Event\ObserverInterface;

use Magento\Catalog\Model\Product;
use Magento\Checkout\Model\Cart;


class salesQuoteItemSetCustomAttribute implements ObserverInterface
{
   protected $_objectManager;

    public function __construct(
        \Magento\Framework\ObjectManagerInterface $objectManager,
        \Magento\Checkout\Model\Cart $cart,
        \Magento\Catalog\Model\Product $product,
        \Magento\Framework\ObjectManagerInterface $interface,
        \Magento\Quote\Model\Quote\Item $quote,
        \CollectionFactory $productCollectionFactory
    ) {
        $this->_objectManager = $objectManager;
        $this->cart = $cart;
        $this->product = $product;
        $this->objectManager = $interface;
        $this->quote = $quote;
    }
    public function execute(\Magento\Framework\Event\Observer $observer)
    {
        $product = $observer->getProduct();
        $quoteItem = $observer->getQuoteItem();
        $quoteItem->setCustomAttribute($product->getCustomAttribute());
    }
}

etc/events.xml

<?xml version="1.0" encoding="UTF-8"?>

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../../../../../../lib/internal/Magento/Framework/Event/etc/events.xsd">

            <event name="sales_quote_item_set_product">
                <observer
         name="product_point_quote"
         instance="Namespace\Modulename\Observer\salesQuoteItemSetCustomAttribute"/>
            </event>
</config>

Create catalog_attributes.xml in etc to convert attribute to quote:

<?xml version="1.0"?>

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Catalog:etc/catalog_attributes.xsd">
    <group name="quote_item">
        <attribute name="custom_attribute"/>
    </group>
</config>

This will bring your attribute on Cart Page