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Actually, each network / router the traffic passes through , has its own MTU (Max transfer unit) which could be the same for all or different. Now, when a host sends a packet of 1500 bytes and there is a router (or routers) that can not handle this, then it is split to smaller ones. If each router will assemble the full packet (original) then re split, then you can imagine the load on the routers. For that it is left to the last point to do so.
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Thank you for the answer, that is exactly what I need. 
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Why does Ipv4 does reassembly at the end point rather than at the next router?<br />
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My guess is that to minimize fragmentation lost during the transmission. By reassembling it at the end points we can detect if there is a packet lost and therefore can ask for that lost packet to be re-transmitted again. If we do it at the next router the fragmentation becomes more complex and the chances that error happens is larger. Is this true? Any other reasons?
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Why does Ipv4 does reassembly at the end point rather than at the next router?

My guess is that to minimize fragmentation lost during the transmission. By reassembling it at the end points we can detect if there is a packet lost and therefore can ask for that lost packet to be re-transmitted again. If we do it at the next router the fragmentation becomes more complex and the chances that error happens is larger. Is this true? Any other reasons?

Ipv4 reassembly

Networking software modules are interfaced with a framework implemented on the machine's operating system that implements the networking functionality of the operating system. The best known frameworks are the TCP/IP model and the OSI model. Systems typically do not use a single protocol to handle a transmission. Instead they use a set of cooperating protocols, sometimes called a protocol family or protocol suite.[9] Some of the best known protocol suites include: IPX/SPX, X.25, AX.25, AppleTalk and TCP/IP. Other protocols indirectly related to networking include the hypertext transfer protocol (HTTP) and its related technologies, Dynamic Host Configuration Protocol (DHCP), Domain Name Server (DNS) and other Internet protocols.

Networking Protocols

Networking is the process of connecting computing devices, peripherals and terminals together through a system that uses wiring, cabling or radio waves that enable their users to communicate, share information and interact over distances. Often associated are issues regarding operating systems, hardware and equipment, cloud and virtual networking, protocols, architecture, storage and management.

Networking

Network Management involves issues that are independent of specific hardware or software, including email policies, upgrade planning, backup scheduling and working with managed service providers for Desktop-As-A-Service (DaaS), Software-As-A-Service (SaaS) and the like through the use of tools, coupled with manufacturer standards, best practice guidelines, policies and procedures plus all other relevant documentation. Network management also includes monitoring, alerting and reporting, management reporting, planning for device or service updates, the backup of configurations, the setting of key performance indicators and measures (KPIs/KPMs), associated service level agreements and problem records as part of the IT Service Management (ITSM) framework.